What word goes in the box with the question mark?
The word is
Each of these is an anagram of a number plus a single extra letter:
It turns out that
there's only one way to fill in the remaining numbers from 1 to 16 to create a magic square (that is, a square where all the rows, columns, and diagonals sum to the same number). That solution is:
05 13 04 12
16 08 01 09
02 10 15 07
11 03 14 06
And the only word that is an anagram of SIX plus an extra letter is AXIS.