An object at rest stays at rest and an object in motion stays in motion with the same speed and in the same direction unless acted upon by an unbalanced force.

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This statement captures inertia—the resistance of a mass to changes in its motion. When no net external force acts, an object at rest stays at rest and an object in motion continues moving with the same speed in the same direction. In other words, velocity remains constant unless something unbalances the forces on the object, which then leads to acceleration according to F = ma. This is Newton's first law, also called the law of inertia. The other options refer to different ideas: Newton's third law is about action–reaction pairs, gravity is a specific force of attraction between masses, and conservation of energy deals with energy transformation and retention in a system.