Imagine you have 4 'slots' on the stage:
_ _ _ _
In the first one there can be 1 out of 9 performers.
In the second - 1 out of 8 (because one has been already chosen).
In the third - 1 out of 7 (because two have been selected).
In the forth - 1 out of 6 (because three are already on the stage).
Total number of combinations is equal to: 9x8x7x6 = 3024
So the correct answer is D.
Hope it helps!
