The answer is 3.
When a ball is thrown into the air, there is no friction (there's nothing rubbing against it), so the only force acting on the ball is gravity, which aims to pull the ball back down. Since this is the only force, the answer will equal the gravity acting on the ball. Since "weight" is defined as just that (the gravity acting on something), the answer is 3, the magnitude of the rock's weight (or the force of gravity).