Which one is correct?

Welcome to modern English. They're all acceptable. 'A' and 'C' might be the better but context might shape yet another as 'better' for that situation. 'A' suggests he is famous for at least two things, where 'C' suggests he is famous for solely on his acting career. 'D' and 'B' seem a tiny bit dismissive. As if the acting career might not have been exactly first flight. Say the difference between Ronnie Howard vs Clint Howard.

Xanina: And why?

Because they are all grammatical, X.

A and B not good, too colloquial.

C and D good.

McTag: A and B not good, too colloquial.
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That wasn't what the OP asked about, but no matter. You can explain your reasoning.

That, if you read my whole post, is exactly what the OP asked about.

And yes, I can explain my reasoning.

McTag: That, if you read my whole post,

Naaaaaa, it was too long and overly complicated.

McTag: And yes, I can explain my reasoning.

Please proceed.

JTT, do us all a favor and use the BBC quote codes.

Yes, all perfectly fine. My personal preference would be C, but the others are OK (which some people trace to "Oll Korrect", i.e. "All Correct", from a 19th century fad for faux-"colloquial" spellings, speaking of correctness).

"Only small minds think there's only way to say something."

Is he famous, or mainly famous?
He is famous.
Why is he famous?
He is famous because of his acting, principally.
He is famous, mainly because of his acting career.
He is famous mainly for having a successful acting career.

Since " mainly" does not modify "famous", I would, strictly speaking, reject A and B although of course you will often see this construct in everyday speech.