The OA is B. I just don't really see it.
S1 is clearly insufficient bceause we can have 2^6 or 4^3, where the value of a is different in each case.
S2 says that n=6, so we know that a^6 has to be a factor of 8!
it might be difficult to see why S2 is sufficient at first, but try out different numbers. 3^6 is not a factor of 8! because you need 6 3's in 8x7x6x5x...etc in order for it to be a factor. but 8! only has 2 6's in it...try it out for 4 and 5, and you'll quickly see that the only possibility for a is 2.