Depends on your context. "Revenge was the main reason for the fire" sounds like someone set the fire FOR THE PURPOSE of getting revenge on someone. "Revenge was the main cause of the fire" sounds like someone was doing something that caused the fire, intentionally or not. They are fairly similar, but do have slightly different connotations depending on what you're trying to say.

reason for anything that produces an effect is reason

If this is an exam question, it's not a very good one. Neither option is quite right, yet neither option is totally wrong. 'Revenge' is a person's motivation for doing something. Presumably we are talking about an arsonist, and this was his or her motivation for committing the crime. 'Revenge' is an emotion. It can't really be the cause or the reason for a fire.