The correct answer to this question is Mass. Inertia is known as Newton's First Law of Motion. This Law comes from Sir Issac Newton, who was a scientist. This Law states that whether an object is moving or at rest, they are at the same speed.
This is until something makes the object change. Inertia is the resistance of an object changing motion. An example is a rubber ball. For that ball to bounce, like a basketball, it must be picked up by a person and thrown onto the floor. Without that occurring, it will stay at rest and not move.