Concept:Use “who” when the person is the subject doing the action, and “whom” when the person receives the action.
Explanation:Look at the clause after the blank: “rescued us.”
Ask: Who rescued us?
The boy rescued us, so “the boy” is the doer of the action.
Since the boy is the subject, we need the subject pronoun “who.”
“Whom” would be used if the boy received the action, like “This is the boy whom we rescued.”
“Which” is wrong because it refers to things or animals, not people.
“Whose” is wrong because it shows possession, not the subject of an action.
Therefore, “who” is the correct relative pronoun.
Answer:D. who