The magnitude of the force that the object exert on the child is 100N(option C)
Newton's third law of motion states that action and reaction are equal but opposite in direction OR it can also be stated as for every action there is an equal but opposite reaction.
This means that If an object A exerts a force on object B, then object B must exert a force of equal magnitude and opposite direction back on object A.
Similarly a child exerts a force of 100N on an object , the object will also exert the same force of 100N on the child but In opposite in direction.
Therefore the force the object exerts on the child is 100N.
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