Final answer:
Motion is always relative to an observer's frame of reference, contradicting the notion of motion being unchanging. This is due to relativity, highlighting the importance of specifying a frame of reference when describing motion.
Step-by-step explanation:
Is motion relative? The correct answer is c. Yes, motion is always relative to an observer's frame of reference. The concept of relativity, first articulated by Galileo and later developed by Einstein, implies that there is no unchanging, universal frame of reference. Instead, motion is observed and measured differently depending on the observer's own state of motion. For instance, a passenger in a car might appear stationary to the driver, but this passenger is in motion relative to an outside observer on the street. The importance of specifying a frame of reference becomes clear when we realize that an object at rest in one frame may be in motion in another. Thus, it can be said that the notion of motion being unchanging is not valid as it is always dependent on the frame of reference selected to analyze it.