C. have been admitted.
It is the present perfect tense. This agrees with the viewpoint in time of the sentence-- which is from the present. "have been admitted" tells of a completed action, but we are talking about it from a position in the present. We want the present because that is what the first part of the sentence is in. You could also say "were admitted", but that is not one of your choices.
"are admitted earlier" does not work-- "are admitted" is in the present, while "earlier" refers to a past time.