But _________ first thought of putting tea leaves into small bags?

• A. Who
• B. What
• C. Which
• D. Where

Answer: (A)

Thinking of the right person is the focus here. The sentence asks who was the first to have that idea, so the word that fits is the pronoun used for a person as the subject of the action “thought.” Who identifies the person responsible for the idea. What would ask about a thing or idea itself, Which would require a defined set to choose from, and Where asks about a place. So the natural, correct choice is the one that points to the person who did the thinking.