Each vowel in the word COMPLAINED is changed to the letter following it in the English alphabetical order and each consonant is changed to the letter preceding it in the English alphabetical order. Which of the following letters will appear twice in the group of letters thus formed?

Vowels are changed to the letter following them in the English alphabet. $$\rightarrow$$ Vowel +1 

Consonants are changed to the letter preceding them in the English alphabet.$$\rightarrow$$   Consonants-1

C -1 →B, O +1→P, M -1 →L, P -1→O, L-1 → K, A+1 →B, I +1→J, N -1→M, E+1 →F, D- 1 →C

The new letters are:
B P L O K B J M F C

The letter that appears twice is B.

Thus, correct option is (c).