It's D, 15!/(15 - 5)! .
15! is the number of ways of permuting 15 objects. In other words, if you have 15 open positions,
_ _ _ _ _ _ _ _ _ _ _ _ _ _ _
from the 15 total objects you can pick, there are 15 choices for the first position, 14 for the second, 13 for the third, and so on, so that there is a total of
15 * 14 * 13 * … * 2 * 1 = 15!
possible arrangements.
But you're taking 5 objects at a time, so we only have 5 positions to fill:
_ _ _ _ _
and there are 15 choices for the first position, 14 for the second, 13 for the third, 12 for the fourth, and 11 for the fifth, so the total number of arrangements is
15 * 14 * 13 * 12 * 11 = 15!/10! = 15!/(15 - 5)!