According to the laws of motion, an object will remain in motion unless acted upon by what?

An object will remain in motion unless acted upon by an external force, as stipulated by Newton's First Law of Motion. This principle, often summarized as the law of inertia, asserts that an object will continue in its state of rest or uniform motion in a straight line unless a net force causes it to change that state.

This means that for any change in motion—whether starting, stopping, or altering direction—there needs to be some force from outside the object that influences its movement. For example, friction, gravity, or a push or pull from another object can all serve as external forces.

In contrast, options such as a constant speed, gravitational pull, or magnetic field do not capture the essence of how objects behave according to Newton's laws. While gravitational pull is indeed a type of external force, it is not the overarching principle governing the inertia of all objects. Instead, it is specific to certain conditions and scenarios. Hence, the most accurate and broad answer to the question is that an object remains in motion unless acted upon by an external force.