I would say C
With for 4 letters you can create: 26 x 26 x 26 x 26 = 26^4 codes
With for 5 letters you can create: 26 x 26 x 26 x 26 x 26 = 26^5 codes
Overall: 26^4 + 26^5 = 26^4 x (1 + 26) = (26^4) x 27 --> C
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