Final answer:
Average acceleration is defined as the change in velocity divided by the time interval in which the change occurs, and it is the correct term that describes the rate of change of velocity.
Step-by-step explanation:
The concept in question is acceleration, which is the rate at which velocity changes over time. When calculating the average acceleration, we use the change in velocity (Δv) over a specific time interval (Δt). Therefore, the average acceleration (aavg) is defined as Δv divided by Δt. In the context of choices provided, the correct answer to 'What is the rate of change of velocity called?' is d. Acceleration.
It's important to note that speed is a scalar quantity that refers to how fast an object is moving, and average speed is the total distance traveled divided by the total time it took to travel that distance. Unlike speed, acceleration can cause a change in the magnitude or direction of velocity, or both.