What best describes a condition when an object is moving at constant speed on a flat surface?

The scenario of an object moving at constant speed on a flat surface indicates that there is no net change in its motion, which means it is in a state of equilibrium. In this context, the net force acting on the object is zero. This occurs when all the forces acting upon it, including any applied force, friction, and gravitational forces, are balanced. When the forces are balanced, the object maintains its speed and direction, consistent with Newton's first law of motion, which states that an object at rest will stay at rest and an object in motion will stay in motion at a constant speed, unless acted upon by a net external force.

In contrast, unbalanced forces would result in acceleration or a change in motion, which contradicts the definition of constant speed. Additionally, the object's weight being greater than the friction does not directly relate to the object moving at constant speed; rather, it is the balance of all forces that determines the motion. Lastly, while the presence or absence of air resistance might affect the forces involved, it is not a requirement for the definition of constant speed on a flat surface. Thus, the statement that the net force acting on the object is zero accurately captures the necessary condition for maintaining constant speed.