Which quantity refers to how fast an object is moving?

The correct answer is speed, as it specifically refers to how fast an object is moving, regardless of its direction. Speed is a scalar quantity, meaning it only has magnitude and does not include any directional information. This is different from velocity, which also measures how fast an object is moving but incorporates the direction of that movement, making it a vector quantity.

Acceleration, on the other hand, describes the rate of change of velocity over time and is related to how quickly an object is speeding up or slowing down. Displacement refers to the change in position of an object and is a vector quantity that considers both distance and direction from the starting point to the endpoint. Therefore, while all these terms are related to motion, speed uniquely identifies the rate of movement without any consideration of direction.