The correct option is
A whom
Answer : A
Explanation:
In the question, the object of the verb (met) is "the man".
The relative pronoun that we use for people are: "who", "that", "whose", "whom"
Thus out of the provided options the correct relative pronoun to fill the blank is "whom".
Relative Pronoun "where" is used to refer to a place,
"when" is used to refer to time,
& "which" is used to refer to objects.
Therefore the correct complete sentence is:
The man WHOM you met at the party was a famous film star.