Explanation:

- Option 1: "whom"

- Used for the object of a verb or preposition.

- Example: The person whom you met yesterday.

- Option 2: "that"

- Used for things or specific cases in informal contexts.

- Example: The book that is on the table.

- Option 3: "who"

- Correct word here.

- Refers to a subject or person.

- Example: This is the girl who won the prize.

- Option 4: "which"

- Typically used for non-human items.

- Example: The car which passed by was red.

- Correct Answer: Option 3 - "who" is appropriate for referring to a person.