Even God couldn't square root 125 to get an integer... :$ you have just made the same mistake as me. Plz do give your ans a brief double-check)
The only possible answer ignoring the last limit of ones digit should be:
n=x^6;
proof:
if n = x^6, then it can always be factorized to (x^2)*(x^3)
then considering the last condition as ones digit is 5
trial and error:
5^6 = 15625
15^6 >10^7
bingo! n = 15625 = 125*125 = 25*25*25!
(ps: the above proof is not n&s, but the tricky here is that GRE Q could only have one correct answer. And even if there are multiple correct answers, filling one correct ans is just OK)