Final answer:
The boy in the car should throw the ball to the right relative to the car to reach the girl standing on the right side of the road. The ball's velocity relative to the ground will be a combination of the velocity of the car and the velocity at which he throws the ball.
Step-by-step explanation:
The question deals with the concept of relative velocity in physics. When the boy in the moving car throws a ball to the right at 20 km/h, and the car is moving at 10 km/h, the velocity of the ball relative to the ground would be the vector sum of the velocity of the ball relative to the car and the velocity of the car relative to the ground. As the motions are perpendicular to each other, we use the Pythagorean theorem to find the magnitude of the resultant velocity.
Therefore, the correct answer is: b) Relative to the car, he should throw the ball to the right, since he wants to get the ball to the girl on the right-hand side of the road. The velocity of the ball relative to the ground would not be simply 10 km/h or 20 km/h, but a combination of both velocities considering directions.