C is correct, for C uses a subject pronoun HE that is necessary for the verb TO BE.
It is she who calls me every evening
If I were he, IтАЩd have to acknowledge it.
Object pronouns HIM/HER are used in the following constructions
(1) I will love no one but him
(2) A user may be sure that no one but him will have access to his files.
(3) He confirms that she is invisible to all but him.
(4) Her real name is unknown, to all but her.
(5) those who live may live not for themselves but for him
Stoylar, I thought that we should look to se who is doing the action to tell if SUBJECT or OBJECT pronoun must be used, am I correct?
In the sentence: "If I were he, IтАЩd have to acknowledge it"
"I" is doing the action, that is, "I" is the subject of the sentence.
Like for instance: "If I were an alien, I would be green and slobby"
In this case "alien" is not part of the subject, isn't it?