We start from the string $ABCDEF$ and we can edit it with the following rules:
1.$ABCDEF \Rightarrow ADBECF$
2.$ABCDEF \Rightarrow DAEBFC$
Which string can't be accessed?
In these kind of problems we usually look for a common thing in every change of the the string, which I can't do here.
According to the book, the answer is