Select the segment of the sentence which has an Adjective clause __________

Who has suggested

The correct answer is Option (3) - Who has suggested

In the sentence "The letter was written in answer to a group of clergyman and leaders who has suggested that he should try to change the tradition," the segment with the adjective clause is:   Who has suggested

An adjective clause modifies a noun or pronoun, providing additional information about it.
In this sentence, "who has suggested" describes the group of clergyman and leaders. It clarifies what this group did by stating their suggestion.
The relative pronoun "who" introduces the adjective clause.