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, I’d 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