From the given options, the statement that best explains why the two scientists proposed different theories about the planets is:
They had different goals.
Kepler focused on studying the speed of the planets, specifically their orbital motion and the patterns they exhibited. His goal was to mathematically describe the observed planetary motion, which led to his three laws of planetary motion.
On the other hand, Newton's goal was to understand the forces that governed the motion of the planets. He developed the law of universal gravitation, which explained that the force of gravity between two objects depended on their masses and the distance between them. Newton's theory provided a physical explanation for the observed motion of the planets.
Since Kepler and Newton had different objectives in their research, their theories focused on different aspects of planetary motion.