Final answer:
The box with a velocity of 5.20 m/s and decelerating at 5.20 m/s² is considered to be decelerating, which is a negative acceleration in the direction of the velocity. Hence, the appropriate description of this motion is C) Decelerating.
Step-by-step explanation:
The student's question pertains to a box with a vertical velocity (vy) of 5.20 m/s and a deceleration (slowing down) at a rate of 5.20 m/s2. The correct characterization of this motion in physics terms is decelerating.
Deceleration is the rate of change of velocity when an object is slowing down, which is shown by a negative acceleration in the direction of the velocity. Since the box has a positive velocity but a negative acceleration, it is decelerating, thus the correct answer is C) Decelerating.
Moreover, the rate of change of velocity is called acceleration. When an object is moving with a positive velocity and slowing down (positive velocity, negative acceleration), acceleration is considered negative in the context of the direction of the object's initial movement.