7. Select the sentence that correctly uses the relative pronoun.

a. Do not ask for who the bell tolls.

O b. I could not tell who's answer was correct.

c. To whom should we send the check.

d. Whose the best cook in the competition?

Answer: C

Explanation: whom is object pronoun which applies to people, which means it describes the person on the receiving end of action that was actually performed, e. g I don't know for whom he called, from here the main clause is (I don't know) and the relative clause (whom he called). The same goes for "To whom should we send the check" to whom (main clause) while "To whom should we" (relative clause), send the check (main clause) ...

C, is where it is properly used.