(b) The weight of the ball is the force of gravity on the ball so the moon is exerting a force of 40 N down hence the ball must be exerting a force of up on the moon.
(The weight of the ball; is; the force of gravity on the ball) (the moon; is exerting; a force of 40 N)
The weight of the ball the moon
is is exerting
the force of gravity on the ball
a force of 40 N