Final answer:
The speed of light in a vacuum is a constant 3.00 × 108 m/s, which is a central concept in Einstein's theory of relativity and remains unchanged regardless of the observer's or the light source's motion.
Step-by-step explanation:
The statement about light in a vacuum that is true is: The speed of light is always constant. In a vacuum, the speed of light is measured as 2.9979 × 108 m/s, approximately 3.00 × 108 m/s. This constancy is a fundamental concept in physics and a cornerstone of Einstein's theory of relativity. The speed of light does indeed vary in different materials due to the index of refraction, but in a vacuum, its speed remains unchanging, regardless of the motion of the source or the observer.