A certain stock exchange designates each stock with one, two or three letter code, where each letter is selected from 26 letters of the alphabet. If the letters maybe repeated and if the same letters used in different order constitute different code, how many different stocks is it possible to uniquely designate with these code.
a) 2951
b) 8125
c) 15600
d) 16302
e) 18278

I was not able to solve. Can someone please help. I will post OA soon.

Thanks.