A certain stock exchange designates each stock with a one-, two-, or a three letter code, where each letter is selected from the 26 letters of the alphabet. If the letters may be repeated and if the same letters used in a different order constitute a different code, how many different stocks is it possible to uniquely designate with these codes?
a) 2,951
b) 8,125
c) 15,600
d) 16,302
e) 18,278
OA follows…