Learn Swedish: Mannen undrar vad som provocerar dig mest, pseudonymen eller signaturen. - The man wonders what provokes you the most, the pseudonym or the signature.

In an indirect question, if the question word (like vad or vem) is the subject doing the action, you must add som. Since "what" is doing the provoking here, it acts as the subject of the embedded clause, so it becomes vad som provocerar.

Would we use som if the sentence was "The man wonders what you provoke"?

No, in that case you would say Mannen undrar vad du provocerar. Because du (you) would be the subject doing the provoking, vad is just the object, meaning the extra som is not required.

Why are pseudonymen and signaturen in the definite form?

They are specific options being presented, matching the English "the pseudonym or the signature". They take the definite -en suffix because they refer to specific, known entities that the speaker and listener are comparing.

Does signatur just mean the physical way you sign your name on a document?

It can mean that, but in Swedish journalism and online spaces, a signatur also refers to a pen name or "handle" used by someone writing an anonymous letter or comment. That is why it is used here as a direct alternative to a pseudonym.