It means they are all equally acceptable.

person or thing that is considered a reasonable target for criticism, exploitation, or attack

Game is a term for animals being hunted by humans. Fair game is a term to describe that a certain "competition" is open to all participants, as opposed to having some conditions or restrictions, which might be unfair game, but we don't use this term. For instance, if you break up with your boyfriend or girlfriend, they are now fair game, and you have nothing to complain about if your friend picks them up. In your example, any of the possible interpretations are reasonable or acceptable options as Ruthi wrote. It's kind of a strange sentence in which to use the term. Other examples of fair game can be your method: In sparring, anything you do with your fists or feet are fair game. So here you could interpret this as 'fair competition'. In war, there are no rules. Everything is fair game. All tactics are fair game.