Final answer:
At equilibrium, there is no net force acting on an object, allowing it to either remain at rest or continue moving at a constant velocity according to Newton's first law of motion. Hence, the correct answer is option (a).
Step-by-step explanation:
The statement that best describes what occurs at equilibrium is: a) No net force acts on an object. At equilibrium in physics, particularly in mechanics, it means that all the forces acting on an object are balanced, resulting in zero net force.
As a consequence, if the object is at rest, it stays at rest, and if the object is in motion, it continues to move at a constant velocity (constant speed in a straight line). This state is described by Newton's first law of motion, which states that an object in motion stays in motion with the same speed and in the same direction unless acted upon by an unbalanced force.
It's important to clarify that equilibrium does not mean that the object is not moving. It can be in motion, but there should be no change in its state of motion, which means no acceleration.