Which of the following is a source of natural background radiation?

Natural background radiation primarily originates from various sources in the environment, including cosmic rays, which are high-energy particles that come from outer space and interact with the Earth's atmosphere. These cosmic rays contribute significantly to the overall exposure humans receive from natural radiation.

In contrast, electric appliances are sources of man-made radiation rather than natural background radiation. Chemical reactions, while they can involve radioactive materials, do not specifically contribute to background radiation in the same way cosmic rays do. Geothermal energy is a form of harnessing heat from the Earth, which can involve some radioactive decay due to naturally occurring radioactive elements within the Earth, but it is not a widely recognized or significant source of background radiation for the general population when compared to cosmic rays.

Thus, the correct identification of cosmic rays as a source of natural background radiation underlines their universal presence and the fact that they are a natural aspect of our environment.