What is a fundamental characteristic of a photon in the context of physics?
A) A photon is a particle of light with no mass and travels at the speed of light in a vacuum.
B) A photon is a charged particle that can change its mass depending on the energy it carries.
C) A photon is a subatomic particle found in the nucleus of an atom, responsible for nuclear reactions.
D) A photon is a type of matter particle that has mass and can be at rest.