Which physical quantity changes when an object’s speed varies?

When an object's speed varies, the most relevant physical quantity that changes is velocity. Velocity is a vector quantity that not only depends on the speed of an object but also its direction. When the speed of an object changes, either increasing or decreasing, or if it changes its direction while maintaining a constant speed, the velocity of the object will change accordingly.

For instance, if a car accelerates from a stop, its speed is increasing, and thus, its velocity changes since both the magnitude (speed) and potentially the direction are involved. This highlights that velocity is a comprehensive measure that reflects both how fast and in which direction an object is moving.

In contrast, magnitude typically refers to the size or amount of something without direction and does not fully encompass the effects of speed variations when direction is also a factor. Scalar quantities, by definition, have only magnitude and not direction, so they do not capture the full nuance of what happens to velocity. Mass, on the other hand, is an intrinsic property of an object and remains unchanged regardless of changes in speed or velocity. Thus, the correct choice emphasizes the dynamism inherent in velocity in response to changes in an object’s speed.