Final answer:
A change in motion, which is acceleration, is caused by a force. This aligns with Newton's first and third laws of motion, indicating that an external force is necessary for any change in motion to occur.
Step-by-step explanation:
Any change of motion is indeed caused by a force. A change in motion can be explained as a change in velocity, which includes changes in speed, direction, or both. According to Newton's first law of motion, a change in motion—essentially an acceleration—occurs only when a net external force acts on an object. Acceleration is defined as the rate of change of velocity, and this can mean speeding up, slowing down, or changing direction. Thus, the correct option for the question 'Any change of motion is caused by a?' is 'a. Force'.
Moreover, the question touches upon concepts such as Newton's third law of motion, which explains how external forces cause changes in the motion of a system and that the rate of change of momentum is directly proportional to the force applied. Additionally, acceleration is also identified as the rate at which velocity changes.