Concept:Relative pronouns are selected based on the noun they refer to and its function in the sentence.
Explanation:The correct choice depends on the complete sentence containing the blank.
"Which" is used for things.
"What" means "the thing which" and does not refer back to a specific noun.
"Who" refers to a person acting as the subject.
"Whom" refers to a person acting as the object.
To pick the best answer, the full sentence is needed.
The given question does not provide that sentence.
Therefore, no single option can be confirmed as correct.
Answer:Cannot be determined because the sentence to be completed is missing.