A40.0 kilogram child exerts a 100 newton force on a 50.0 kilogram object the magnitude of the force that the object exerts on the child is?
a) 0.0n
b) 80.0n
c) 100n
d) 125n

The answer is choice C.

Explanation:

According to Newtons third law of motion, the force that object exerts on the child is equal and opposite to the force that the child exerts on the object:

Thus if the child exerts a force of 100 newtons on the object, the object must also exert a force of 100 newtons on the child but in the opposite direction.