Acceleration is defined as:

The definition of acceleration refers to the rate of change of velocity. This means it describes how quickly an object's velocity is increasing or decreasing over time. Velocity itself is a vector that includes both speed and direction. Therefore, when acceleration is considered, it takes into account not just changes in speed but also any changes in direction.

For instance, if a car speeds up as it moves straight down the road, its velocity is changing, and thus it is experiencing acceleration. Similarly, if that car turns a corner while traveling at a constant speed, there is still acceleration occurring because the direction of its velocity is changing. This comprehensive view aligns perfectly with the definition provided in the correct answer, illustrating that acceleration encompasses both aspects of velocity change.

The other options do not accurately define acceleration. The first choice suggests that velocity remains constant, which does not involve acceleration at all. The third choice incorrectly defines acceleration solely with speed and time, leaving out the critical aspect of change in velocity. The fourth choice confuses distance with acceleration, as it describes a different relationship involving distance covered over time rather than the change in velocity.