Manju ..please use the search option before posting a new question ..

here goes the answer its C

Max number of 4 letter code out of 26 is 26^4
similarly max number of letter code is 26^5

As the question uses the word"OR".. we need to add the above two ..

there fore its 26^4 + 26^5

take 26^4 common

26^4 ( 1+ 26)

hence its 27* 26^4.

hopes its clear..