Select the segment of the sentence which has an Adjective clause ____…

The correct answer is Option (3) → Who had suggested

An adjective is a word that describes a noun or a pronoun, for example a tall tree, the food is tasty.

An adjective clause (relative clause) describes a noun or pronoun, like an adjective, and usually begins with a relative pronoun: who, whom, whose, which, that. In this sentence, "The letter was written in answer to a group of clergymen and leaders who had suggested that he should try to change the tradition", the clause beginning with who describes the clergymen and leaders.

Option 1: The letter was written in answer — Main clause. It has its own subject and verb and stands alone.

Option 2: To a group of clergymen and leaders — Phrase. It has no verb. It completes "in answer".

Option 3: Who had suggested — Adjective clause. It begins with the relative pronoun who and describes the noun "clergymen and leaders".

Option 4: That he should try to change the tradition — Noun clause. It is the object of "suggested" (suggested what?), so it does the work of a noun.

Contrast: who had suggested describes the noun "clergymen and leaders" (adjective clause). That he should try to change the tradition is the object of "suggested" (noun clause).