Tom is grateful that he is/has been/was admitted to the school.

[emsr2d2]

I'd use b) in that situation.

[GoesStation]

C is also possible, but I agree that B is the most likely.

[kadioguy]

They're all grammatical and natural but they obviously have different uses, since they use different tenses.

Which one is best depends on exactly what you want to focus on—present, past, or future.

jutfrank, could you please tell me in what use you think of (a) is grammatical and natural? (This is definitely the last question of the thread, please. )

[jutfrank]

jutfrank, could you please tell me in what use you think of (a) is grammatical and natural?

(a) and (b) are much the same, since they both use the present tense to talk about Tom's current state.

I agree with the others that (b) is a more likely utterance. I can't easily think of a context where (a) would be preferable.