TT1-36

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)