When a vehicle maintains a constant speed, what is the acceleration?

When a vehicle is maintaining a constant speed, it means that its velocity is not changing over time. Acceleration is defined as the change in velocity over a period of time. Since there is no change in the vehicle's speed while it is moving at a constant rate, the acceleration is effectively zero.

This concept is fundamental in physics: any constant velocity implies that the object is not experiencing any net force causing it to accelerate. Therefore, under these conditions, the acceleration is maintained at zero until an external force acts upon the vehicle to change its speed or direction.

Other alternatives such as positive, negative, or variable acceleration imply some degree of change in velocity, which is not applicable when the speed is constant. Hence, zero is the only appropriate answer.