Answer:
C.

Explanation:
The stocks can be coded using either 4 letters or 5 letters.
Maximum possibilities for 4 letter codes:
Since any letter from 26 alphabets can be used, there are 26 ways to chose each of the 4 letters of the code.
Therefore, total number of ways to form the code will be = 26 x 26 x 26 x 26 =

Maximum possibilities for 5 letter codes:
Based on the same logic as used in previous part, there are 26 ways to chose each of the 5 letters of the code.
So, total number of ways to form the code will be = 26 x 26 x 26 x 26 x 26 =

Total maximum possible ways to code:
The total maximum possible codes will be the sum of possible 4 digit and 5 digit codes.
So, maximum number of codes =

Therefore, option C gives the correct answer of maximum number of different stocks that can be designated with these codes.