Explanation:
The relative pronoun must refer to "The boy" (Person) and act as the subject of the verb "sits".

Who: Used for people (Subject case). Correct.
Which: Used for things/animals.
Whom: Used for people (Object case).
Whose: Possessive.

"The boy who sits..." creates an Adjective Clause describing the boy.

Detailed Topic Insight (Adjective Clauses):
Also called Relative Clauses.

Defining (Restrictive): Essential information. "The man who stole my bag was arrested." (No commas).
Non-Defining (Non-Restrictive): Extra information. "My father, who is 60, loves gardening." (Uses commas).

Clauses

Detailed Solution

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Solution