I would have said fighting battles. I guess they could arrange, set or run battles, but I never would have thought about it. Could anyone give me an example where “set battle” would be used please?

I think the answer is A. B and D can have the same meaning, and I'm not sure of C. Also, the context you provided doesn't support B, C, or D.

In the game, players create avatars in whose guise they spend hours with other players (28)___________ battles. “There’s lots of working together,” says Alexander. “That’s what made it so attractive, because the social thing was something I always had trouble with. It was a lot easier to socialize and make friends online than it was in real life.”

More context is required.

The correct answer will depend on the context. Could you give more information about where this sentence is appearing? Is it trying to get you to answer a question about a text?