Newton's first law of motion states that a body in motion tends to remain in motion unless acted upon by what?

Newton's first law of motion, often referred to as the law of inertia, asserts that an object will maintain its state of motion—whether at rest or moving at a constant velocity—unless influenced by an external force. This fundamental principle highlights the concept of inertia, which is the tendency of an object to resist changes to its state of motion.

The correct answer emphasizes the role of an external force, which can arise from various sources such as friction, gravity, contact forces, or any other interaction that causes a change in the motion of the object. For example, if a rolling ball encounters friction or a bump, that external force will act on the ball, eventually causing it to stop or change direction.

In contrast, the other options suggest specific scenarios or forces that can act on objects but do not encompass the broader application of Newton's first law. Gravitational pull, electrical currents, and frictional forces can certainly act as external forces, but they do not individually capture the entire concept as stated in the first law. The essence of the law pertains to any external influence on a body in motion, making the understanding of external forces critical in the context of physics and motion.