What general rule can be concluded about the force needed to keep an object in motion?

The principle behind the idea that the applied force must equal the resistive force relates to Newton's first law of motion, which states that an object in motion will remain in motion unless acted upon by an unbalanced force. For an object moving at a constant velocity, the net force acting on it must be zero. This means that the applied force must counterbalance any resistive forces, such as friction or air resistance.

When an object is in motion and experiencing resistive forces, to maintain its constant speed, the applied force needs to be equal to the total resistive force acting on the object. If there is no additional acceleration or deceleration, this balance ensures that the object continues to move uninterrupted. Therefore, the correct answer highlights the necessity of matching the applied force to the resistive forces to sustain motion without changing velocity.

While other options may suggest certain relationships between applied forces, they don't accurately reflect the balance necessary for maintaining constant motion against resistive forces.