Hi there!
(A) The force that the ball exerts on Earth is equal in magnitude to the force that Earth exerts on the ball.
Gravitational force is an example of an action-reaction pair:
[tex]F_{1,2} = -F_{2,1}[/tex]
Gravitational force is given by the equation:
[tex]F_g = \frac{Gm_1m_2}{r^2}[/tex]
Plugging in the masses of the Earth and ball, gravitational constant, and radius will result in the SAME GRAVITATIONAL FORCE exerted by both objects on each other.