Kepler and Newton studied the movement of planets. Kepler studied the speed of the planets while Newton studied the forces that kept the planets in place. Which of these statements best explains why the two scientists proposed different theories about the planets?

They had different goals.

They studied different models.

They used different sets of data.

They misinterpreted their findings.

As the question said, Kepler studied the speed of planet, which is referred in the Kepler's Third Law. While Newton studied force of the planet, which is referred in the Newton's Second Law.

Then you should investigate the relationship between speed and force.

And they do have a relationship, which is called momentum.

The general law to conclude these laws is the conservation of angular momentum in planet movement, you can search for more details.

So I would say they might use different models or the different set of data, depending on how you understand this question. My answer will be they use different physical quantities.