Answer: a. gravity
Explanation:
From Newton's first law, we know that an object remains in the state of rest or motion, unless and until acted upon by an external unbalanced force. Moon revolves about Earth. The gravitational pull of the Earth keeps the Moon in the orbit. Without gravity, the moon would be traveling in a straight line.
Thus, If there were no a. gravity inertia would cause the moon to travel in a straight line.