Which of the following best describes acceleration?

Acceleration is defined as the rate at which an object's velocity changes over time. This means that if an object speeds up, slows down, or changes direction, it is experiencing acceleration. The concept is grounded in the understanding that velocity itself is a vector quantity, meaning it includes both speed and direction. Thus, whenever there’s a change in velocity (which could be an increase or decrease in speed or a change in direction), acceleration occurs.

This understanding aligns perfectly with the correct choice, as it articulates that acceleration is specifically about the change in velocity measured during a specific time interval. In contrast, options that address distance traveled, force, and mass do not pertain to the definition of acceleration. Distance per unit time relates to speed, force divided by mass pertains to Newton's second law defining acceleration under certain circumstances, and a change in mass does not relate to motion directly. Thus, the first description captures the essential nature of acceleration effectively.