Which of the following does not end with ff?

The correct answer is: Boy

The most frequent double letter combinations are ee, ll, ss, oo, tt, ff, rr, nn, pp, and cc. The double letters that occur most commonly at the end of words are ee, ll, ss, and ff.  There is two words 'boy' and 'scruff'. Only the word 'scruff' has double letter 'ff'. So, the word 'boy' does not end with 'ff'.