What does the concept of inertia explain in terms of physical behavior?

Inertia is a fundamental concept in physics that refers to the tendency of an object to resist changes in its state of motion. According to Newton's First Law of Motion, an object at rest will stay at rest unless acted upon by an external force. This is precisely what option A describes: it highlights the behavior of a stationary object, emphasizing that it will remain in that state unless influenced by an outside force.

Inertia is directly related to mass; the greater the mass of an object, the greater its inertia, making it more resistant to changes in motion. This idea is crucial in understanding how objects behave in both static and dynamic scenarios. The other options delve into related but distinct concepts: the effect of mass on distance covered addresses the influence of mass on movement rather than on rest, speed in a straight line focuses on velocity rather than inertia, and the relationship between force and acceleration pertains to Newton's Second Law, which is broader in scope. Therefore, recognizing that inertia specifically pertains to the tendency of stationary objects to remain at rest underscores the accuracy of option A.