In the territorial behavior of the stickleback fish, the red belly of one male that elicits attack from another male is functioning as

The answer for the question above is a sign stimulus.

Tibbergen used crude models of sticklebacks to investigate which features of male and female sticklebacks elicited attack and courtship behavior from male sticklebacks. The findings were that the male sticklebacks responded in a relatively invariant way and attacked a model with red belly.