I can't figure out one part of this problem..help :[?

Question

I figured out part a and part b. I cant figure out what to put in part c (This answer). My professor said its a word(s) that may appear somewhere in the question. I still cant figure it out.

here is the full problem;

http://i25.tinypic.com/2zgusjm.jpg

any help is appreciated..or thoughts

thanks!

Answer 1

I think what the prof is saying (and his grammar is really bad)

because it "differs from every other number on the list by one digit"