When a boy pushes down on a diving board, the reaction force is the upward force exerted by the diving board on the boy. According to Newton's third law of motion, for every action, there is an equal and opposite reaction. In this case, the action is the boy pushing down on the diving board, and the reaction is the diving board pushing up on the boy. This reaction force is what propels the boy into the air when he jumps off the diving board.