Photon is the basic unit/quantity of?

Photon is the basic unit/quantity of light (option A). In physics, a photon is defined as a quantum of electromagnetic radiation. It is a fundamental particle that carries energy proportional to its frequency. Photons are the force carriers of the electromagnetic force and play a crucial role in the interactions of light with matter. When photons are emitted or absorbed by atoms or molecules, they can cause transitions between energy levels, resulting in phenomena such as emission spectra and the photoelectric effect. Despite being massless, photons exhibit properties of both particles and waves, behaving like discrete packets of energy in some situations and propagating as electromagnetic waves in others. This dual nature of photons is a cornerstone of quantum mechanics and has implications ranging from the behavior of light in optical devices to the understanding of fundamental interactions in the universe.