Question

what is the only force acting on an object orbiting earth?
air resistance
friction
inertia
gravity

Explanation:

When an object is orbiting Earth, in the ideal case (ignoring very - thin upper - atmosphere effects in some low - Earth orbits), air resistance is negligible. Friction is not relevant in the context of orbital motion in space. Inertia is a property of matter (the tendency of an object to resist changes in its state of motion), not a force. Gravity is the force that provides the centripetal force required for the object to orbit Earth.

Answer:

gravity