Each signal that a certain ship can make is comprised of 3 different flags hanging vertically in a particular order.  How many unique signals can be made by using 4 different flags?

A.      10

B.      12

C.      20

D.     24

E.      36

It's a permutation case:

nPr = n!/(n-r)!

We need to choose 3 flags from the available 4 and arrangement in a specific order matters. So, it's 4p3 = 4! = 24 signals

So, the answer is (D)