Adjective clauses (also known as relative clauses) provide additional information about a noun or pronoun in a sentence. Here's the reason for selecting sentences with adjective clauses:

B. "Sunil was the student who won the gold medal." - This sentence contains the adjective clause "who won the gold medal," which provides more information about the noun "student."

D. "The stick with the metal handle belongs to Rishika." - This sentence contains the adjective clause "with the metal handle," which describes the stick, and "belongs to Rishika," which provides additional information about ownership. Both are adjective clauses.