Acceleration indicates:

Acceleration is fundamentally defined as the rate of change of velocity of an object over time. It not only considers how quickly an object is speeding up or slowing down but also takes into account any changes in direction. For instance, if a car increases its speed from 20 m/s to 50 m/s over a span of 5 seconds, it is experiencing positive acceleration. Similarly, if the car is moving in a circular path, it might be maintaining a constant speed while still accelerating due to the change in direction.

The other options present elements associated with motion but do not capture the comprehensive meaning of acceleration. While speed is a component of motion, it does not encompass how that speed is changing over time, nor does it describe the intricacies of direction changes that can occur. Constant speed simply means there is no acceleration taking place. Direction of motion is important but refers more to the vector component of velocity rather than the rate at which velocity itself is changing. Therefore, the definition indicating acceleration as the change in velocity over time most accurately defines what acceleration is.