Question

what is the origin of cosmic background radiation?
it is emitted by stars.
it is a remnant of the big bang.
it is emitted by radio galaxies.
it is dark energy.

Explanation:

Cosmic background radiation is a remnant of the Big Bang. After the Big Bang, the universe was extremely hot and dense. As it expanded and cooled, photons (light particles) were released. These photons have been traveling through space ever since, and they are now observed as cosmic background radiation. Stars emit different types of radiation (like visible light, ultraviolet, etc.) which is not the same as cosmic background radiation. Radio galaxies emit radio - frequency radiation due to processes in the galaxy (like jets from supermassive black holes), which is not the same as the relic radiation from the Big Bang. Dark energy is a theoretical concept to explain the accelerated expansion of the universe and is not the same as cosmic background radiation.

Answer:

It is a remnant of the Big Bang.