Each vowel in the word OPTICAL 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?

 Given:

Word OPTICAL

The given word is 'OPTICAL'
The pattern followed here is:
1) Each vowel is changed to the letter following it in the English alphabetical order → Positional value of vowel +1
2) Each consonant is changed to the letter preceding it in the English alphabetical order → Positional value of Consonant - 1

We can see that the alphabet 'B' appears twice.
Thus, correct option is (d).