Which one is incorrect?
Answer A
Despite the pervasive use of "Whom should I say is calling?", it's flat out incorrect. "Whom" is used to replace the object of a question or statement (e.g., "From whom were you expecting a phone call?") "Who", on the other hand, is used in place of the subject of a question or statement (i.e., the person performing the action in the clause). Here, the action is "calling", so the correct pronoun is "who". One handy trick if you're stuck is to rephrase the sentence using she/her. If "she" is right, "who" is correct; if "her" is right, go with "whom".
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