I don't think GMAT tests you on remainder theorem and I believe you need them to solve question like these.

2^256 = 16^64 = (17-1)^64 when divided by 17 leaves a remainder of (-1)^64 = 1

Also, 2^256 is even, any even number divided by 17(odd) leaves an odd remainder. Only option A is odd. Ofcourse in this case D could be a possibility, but A would be a better guess. Nevertheless, don't bother too much about complicated remainder problems, I haven't seen any official question based on remainder theorems.