Answer: A: a ray of visible light
Explanation:
Light is an electromagnetic radiation. It can travel through vacuum as well as medium. The speed of light is maximum in vacuum i.e. 3 × 10⁸ m/s. From experiments, scientists know that nothing can exceed the speed of the light i.e this speed of light is the upper limit and o particle or wave can travel with speed greater than this. Thus, a ray of visible light would have the greatest speed when observed by an astronaut.