Final answer:
The option that is NOT one of Kepler's laws is 'The gravitational force between two objects varies inversely with the distance between them,' which is actually Newton's law of universal gravitation.
Step-by-step explanation:
The answer to the question of which of the following is NOT one of Kepler's laws of planetary motion is option b: The gravitational force between two objects varies inversely with the distance between them. This description relates to Newton's law of universal gravitation, not Kepler's laws.
Kepler's First Law states that the orbit of each planet around the Sun is an ellipse with the Sun at one focus. Kepler's Second Law, the Law of Equal Areas, posits that a line drawn from a planet to the Sun sweeps out equal areas during equal intervals of time. Lastly, Kepler's Third Law indicates that the square of a planet's orbital period is proportional to the cube of the semi-major axis of its orbit.
Therefore, option b does not describe one of Kepler's laws but is instead attributed to Isaac Newton's formulation of gravitation.