Which could be an example of a situation where acceleration occurs?

Acceleration is defined as a change in the velocity of an object over time. This includes changes in speed (increasing or decreasing) as well as changes in direction. In the context of the provided options, a bike slowing down to stop is a clear example of acceleration because the bike is experiencing a decrease in speed, which signifies a change in its velocity.

When an object is slowing down, it is undergoing negative acceleration (deceleration), indicating that its speed is changing from a higher value to a lower value until it eventually stops. This negative change in velocity is what differentiates the situation from the others, where the objects either maintain a constant speed or with a fixed height or direction—none of which represent a change in velocity that constitutes acceleration.