The answer is:
Idiotic, which has length 7>5 and 7 is a prime number
The text is:
Once upon a time, a carpenter made a peanut butter and pickle sandwich for lunch, but his idiotic kitten ate it.
Which word in this story has length a prime number bigger than five?
Changing key with translation:
|ABCDEFGHIJKLMNOPQRSTUVWXYZ|
|abcdefghijklmnopqrstuvwxyz|
O
|ABCDEFGHIJKL M N OPQRSTUVWXYZ|
|abcdefghijkl>n<>m<opqrstuvwxyz|
Once
| A BCDEFGHIJKLMNOPQRST U VWXYZ|
|>u<bcdefghijklnmopqrst>a<vwxyz|
Once upon
|ABCDEFGHIJKLMN O PQRST U VWXYZ|
|ubcdefghijklnm>a<pqrst>o<vwxyz|
Once upon a
|ABCDEFGHIJKLMNOPQ R S T UVWXYZ|
|ubcdefghijklnmapq>t<s>r<ovwxyz|
Once upon a time,
|A B CDEFGHIJKLMN O PQRSTUVWXYZ|
|u>a<cdefghijklnm>b<pqtsrovwxyz|
Once upon a time, a ca
|ABCDEFGHIJKLMNOPQR S T UVWXYZ|
|uacdefghijklnmbpqt>r<>s<ovwxyz|
Once upon a time, a carpenter mad
|ABCD E FGHIJ K LMNOPQRSTUVWXYZ|
|uacd>k<fghij>e<lnmbpqtrsovwxyz|
Once upon a time, a carpenter made
|A B CDEFGHIJKLMN O PQRSTUVWXYZ|
|u>m<cdkfghijelna>b<pqtrsovwxyz|
Once upon a time, a carpenter made a pe
|ABCDEFGHIJKLM N OPQRSTUV W XYZ|
|umcdkfghijeln>w<bpqtrsov>a<xyz|
Once upon a time, a carpenter made a peanut
|ABCDEFGHIJKL M N O PQRSTUVWXYZ|
|umcdkfghijel>b<w>n<pqtrsovaxyz|
Once upon a time, a carpenter made a peanut butter and
|ABCDEFGHIJKLM N O P QRSTUVWXYZ|
|umcdkfghijelb>p<n>w<qtrsovaxyz|
Once upon a time, a carpenter made a peanut butter and pickle
|ABCDEF G HIJKLMNOPQRS T UVWXYZ|
|umcdkf>s<hijelbpnwqtr>g<ovaxyz|
Once upon a time, a carpenter made a peanut butter and pickle s
| A BCDEFGHIJKLMNOPQRSTUV W XYZ|
|>a<mcdkfshijelbpnwqtrgov>u<xyz|
Once upon a time, a carpenter made a peanut butter and pickle sandwich
|ABCDE F GHIJK L MNOPQRSTUVWXYZ|
|amcdk>l<shije>f<bpnwqtrgovuxyz|
Once upon a time, a carpenter made a peanut butter and pickle sandwich for lunc
|ABCDEFG H IJKLMNO P QRSTUVWXYZ|
|amcdkls>w<ijefbpn>h<qtrgovuxyz|
Once upon a time, a carpenter made a peanut butter and pickle sandwich for lunch,
|ABCDEFGHIJKL M NOPQRS T UVWXYZ|
|amcdklswijef>g<pnhqtr>b<ovuxyz|
Once upon a time, a carpenter made a peanut butter and pickle sandwich for lunch, but hi
|ABCD E F G HIJKLMNOPQRSTUVWXYZ|
|amcd>s<l>k<wijefgpnhqtrbovuxyz|
Once upon a time, a carpenter made a peanut butter and pickle sandwich for lunch, but his
|ABCDE F GH I JKLMNOPQRSTUVWXYZ|
|amcds>i<kw>l<jefgpnhqtrbovuxyz|
Once upon a time, a carpenter made a peanut butter and pickle sandwich for lunch, but his idiotic ki
|ABCDEFGHIJKLM N OPQ R STUVWXYZ|
|amcdsikwljefg>t<nhq>p<rbovuxyz|
Once upon a time, a carpenter made a peanut butter and pickle sandwich for lunch, but his idiotic kitten
| A BCD E FGHIJKLMNOPQRSTUVWXYZ|
|>s<mcd>a<ikwljefgtnhqprbovuxyz|
Once upon a time, a carpenter made a peanut butter and pickle sandwich for lunch, but his idiotic kitten ate
|ABCDE F GHIJ K LMNOPQRSTUVWXYZ|
|smcda>e<kwlj>i<fgtnhqprbovuxyz|
Once upon a time, a carpenter made a peanut butter and pickle sandwich for lunch, but his idiotic kitten ate it.
W
|ABCDEFGHIJKLMNO P QRSTUVWX Y Z|
|smcdaekwljifgtn>y<qprbovux>h<z|
Once upon a time, a carpenter made a peanut butter and pickle sandwich for lunch, but his idiotic kitten ate it.
Which
|ABCDEFG H IJKLMNOPQRSTUVWX Y Z|
|smcdaek>h<ljifgtnyqprbovux>w<z|
Once upon a time, a carpenter made a peanut butter and pickle sandwich for lunch, but his idiotic kitten ate it.
Which word
|AB C DEFGHIJ K LMNOPQRSTUVWXYZ|
|sm>i<daekhlj>c<fgtnyqprbovuxwz|
Once upon a time, a carpenter made a peanut butter and pickle sandwich for lunch, but his idiotic kitten ate it.
Which word in th
|AB C DEFGHIJKLMN O PQRSTUVWXYZ|
|sm>n<daekhljcfgt>i<yqprbovuxwz|
Once upon a time, a carpenter made a peanut butter and pickle sandwich for lunch, but his idiotic kitten ate it.
Which word in this
| A BCDEFGHIJKLMNOP Q RSTUVWXYZ|
|>q<mndaekhljcfgtiy>s<prbovuxwz|
Once upon a time, a carpenter made a peanut butter and pickle sandwich for lunch, but his idiotic kitten ate it.
Which word in this stor
|ABCD E FGHIJKLMNO P QRSTUVWXYZ|
|qmnd>y<ekhljcfgti>a<sprbovuxwz|
Once upon a time, a carpenter made a peanut butter and pickle sandwich for lunch, but his idiotic kitten ate it.
Which word in this story ha
|ABCDEFGHIJKLMNOP Q RS T UVWXYZ|
|qmndyekhljcfgtia>b<pr>s<ovuxwz|
Once upon a time, a carpenter made a peanut butter and pickle sandwich for lunch, but his idiotic kitten ate it.
Which word in this story has leng
|ABCDEFGHIJKLM N OPQRS T UVWXYZ|
|qmndyekhljcfg>s<iabpr>t<ovuxwz|
Once upon a time, a carpenter made a peanut butter and pickle sandwich for lunch, but his idiotic kitten ate it.
Which word in this story has length
|ABCDEF G HIJKLMNO P QRSTUVWXYZ|
|qmndye>a<hljcfgsi>k<bprtovuxwz|
Once upon a time, a carpenter made a peanut butter and pickle sandwich for lunch, but his idiotic kitten ate it.
Which word in this story has length a p
|AB C DEFGHIJKLMNOPQR S TUVWXYZ|
|qm>r<dyeahljcfgsikbp>n<tovuxwz|
Once upon a time, a carpenter made a peanut butter and pickle sandwich for lunch, but his idiotic kitten ate it.
Which word in this story has length a prime num
|ABCDEFGHIJKL M NOP Q RSTUVWXYZ|
|qmrdyeahljcf>b<sik>g<pntovuxwz|
Once upon a time, a carpenter made a peanut butter and pickle sandwich for lunch, but his idiotic kitten ate it.
Which word in this story has length a prime number bi
|ABCDEFGHIJKLM N OP Q RSTUVWXYZ|
|qmrdyeahljcfb>g<ik>s<pntovuxwz|
Once upon a time, a carpenter made a peanut butter and pickle sandwich for lunch, but his idiotic kitten ate it.
Which word in this story has length a prime number bigger tha
|AB C DEFGHIJKLMNOPQR S TUVWXYZ|
|qm>n<dyeahljcfbgiksp>r<tovuxwz|
Once upon a time, a carpenter made a peanut butter and pickle sandwich for lunch, but his idiotic kitten ate it.
Which word in this story has length a prime number bigger than
|ABCDEFGH I JK L MNOPQRSTUVWXYZ|
|qmndyeah>f<jc>l<bgiksprtovuxwz|
Once upon a time, a carpenter made a peanut butter and pickle sandwich for lunch, but his idiotic kitten ate it.
Which word in this story has length a prime number bigger than five?