Final answer:
Velocity most closely illustrates motion by relating to a reference point, as it includes both speed and direction, unlike speed, distance, or acceleration which lack this combined characteristic.
Step-by-step explanation:
The concept that most closely illustrates motion by relating it to a reference point is C) Velocity. Velocity differs from speed as it is not only how fast something is moving, but also includes the direction of the movement relative to a reference point, making it a vector quantity. By comparison, speed is a scalar quantity that does not take into account direction, while acceleration is the rate at which velocity changes. Distance represents how far an object has moved, but also does not include directional information.
Acceleration, as defined in one of the check your understanding questions, is the rate at which velocity changes. A velocity vs. time graph can show you acceleration, the direction of motion, and provide a reference frame for the motion. A position vs. time graph can show you information about the uniformity of motion, changes in velocity, and the displacement of the object. When discussing motion, understanding these concepts and how they relate to each other helps describe the movement of an object or person.