Final answer:
Newton deduced that the Moon must be experiencing a force that pulls it away from a straight-line motion in its orbit around Earth. This force is gravitational attraction and is an example of Newton's laws of motion and universal gravitation at work.
Step-by-step explanation:
Sir Isaac Newton concluded that a force was acting on the Moon, and this force was necessary to prevent the Moon from moving in a straight line, which is what it would do if no force were present. This concept aligns with Newton's understanding of gravitational forces and motion. The correct option from the ones listed is c. a force is needed to pull the Moon away from straight-line motion.
Newton's insight led to the realization that not only does Earth exert a gravitational pull on the Moon but also that the Moon exerts an equal and opposite force on Earth. This mutual interaction is in accordance with Newton's third law of motion, demonstrating that the forces are equal in magnitude but opposite in direction. It's also why Earth does not remain stationary as the Moon orbits; Earth is also affected by the Moon's gravity.
Furthermore, the presence of tides on Earth provides clear evidence of the Moon's gravitational effect on our planet, which was a significant argument in support of Newton's theory of universal gravitation.