Final answer:
The object speeding up has the greatest acceleration magnitude because that is the only scenario in which there is an actual increase in velocity, which defines acceleration.
Step-by-step explanation:
The student is asking about the comparative magnitudes of acceleration for different scenarios involving motion. Acceleration is defined as the rate at which velocity changes. When an object is moving at a constant speed in a straight line (uniform linear motion), its acceleration is zero because the velocity does not change. An object at rest also has zero acceleration since there is no change in velocity. For an object that is slowing down or speeding up, there is a change in the speed, which indicates that there is acceleration.
Therefore, among the given options, the object speeding up (d) has the greatest acceleration magnitude because it is the only scenario where there is an actual increase in velocity. An object slowing down (c) also has acceleration, but in the context of the question, the greatest acceleration refers to the increase in speed.
It's important to note that three ways an object can accelerate include speeding up, slowing down, or changing direction. This means acceleration isn't just about changes in speed, but also changes in the direction of motion, both resulting in a change in velocity.