If f(4) = 0 and f(6) = 6, which of the following could represent f(x) ?

Hmm, well gee, I can think of about a million possibilities.

I don't know what your choices are (since you seem to have forgotten, eh hem, that it was a multiple choice question), but just plug 4 in for x for all of them and see if that equals 0. Then plug 6 in for x for all of them and see if that equals 6. Whichever one works is the answer.