This question is based on the words given below.
 (Left) ACT BUS CAR DAD (Right)
 In each of the words, each vowel is changed to the letter immediately following it in the English alphabetical order and each consonant is changed to the letter immediately preceding in the English alphabetical order. In how many letter clusters thus formed, will no vowel appear?

Given: ACT BUS CAR DAD

According to the question:

In each of the words, each vowel is changed to the letter immediately following it in the English alphabetical order and each consonant is changed to the letter immediately preceding in the English alphabetical order.

Now, changing each letter in the words:

ACT: A → B, C → B, T → S

BUS: B → A, U → V, S → R

CAR: C → B, A → B, R → Q

DAD: D → C, A → B, D → C

Thus, the new clusters formed are:

ACT → BBS

BUS → AVR

CAR → BBQ

DAD → CBC

In these clusters, the ones without any vowel (A, E, I, O, U) are:

BBS, BBQ, CBC

Hence, the number of letter clusters where no vowel appears is: 3

Therefore, the correct answer is "Option 3".