Here's why:
* Adjective Clauses: These clauses function like adjectives, modifying nouns or pronouns. They usually begin with a relative pronoun (who, whom, whose, which, that).
* In this sentence: The clause modifies the noun "gentleman". It tells us *which* gentleman we are talking about, specifically the one who was sitting close to them.
The complete sentence can be broken down like this:
* The gentleman (noun)
* who was sitting close to them (adjective clause modifying "gentleman")
* [is a] senator from their home state (predicate)