Answer :
Whom did Carmen call?
The reason for it is that "whom" is the object of the sentence. You use "whom" when you can replace the word with "her/him" when answering the question, so-- whom did Carmen call? She called him/her.
The reason for it is that "whom" is the object of the sentence. You use "whom" when you can replace the word with "her/him" when answering the question, so-- whom did Carmen call? She called him/her.