What is defined as the rate of change of displacement?

The rate of change of displacement is defined as velocity, making it the correct choice. Displacement is a vector quantity that refers to the change in position of an object, while velocity is also a vector quantity that indicates both the speed of the object and the direction in which it is moving.

To be more precise, velocity is calculated as the change in displacement divided by the time over which that change occurs. This means that it takes into account how far an object has moved in a specific direction during a particular time interval. Therefore, when discussing motion with direction, velocity gives a complete picture of how an object's position changes over time.

In contrast, speed, while also related to motion, is a scalar quantity that only considers how fast an object is moving, regardless of the direction. Acceleration refers to the rate of change of velocity, not displacement, and momentum is the product of mass and velocity, which relates to an object's motion but does not directly address the change in displacement. Thus, velocity is the term that uniquely encapsulates the concept in the question.