Final answer:
A planet's gravity is related to its mass and size, as described by Newton's universal law of gravitation. This gravitational force determines the weight of objects on the planet's surface and the orbits around it.
Step-by-step explanation:
The gravity of a planet is related to its mass and size, among other factors. Specifically, a planet's gravitational force is proportional to its mass and inversely proportional to the square of the distance from its center, according to Newton's universal law of gravitation. Gravity keeps planets in orbit, and it affects the weight of objects on the planet's surface. Therefore, the correct answer to the student's question is C. Mass and size. It's important to remember that while an object's mass remains constant regardless of location, its weight can vary depending on the local gravitational force.