Which radiation exposure is considered a naturally occurring source?

Cosmic radiation is the correct answer because it originates from outer space and constantly bombards the Earth's atmosphere. This type of radiation is a natural component of our environment and contributes to the background radiation levels that all humans are exposed to throughout their lives. Cosmic rays can reach the Earth and include high-energy particles that originate from outside the solar system, such as those produced by supernovae and other cosmic phenomena.

In contrast, medical x-rays, industrial radiography, and radiation from nuclear power plants are all artificial sources of radiation. Medical x-rays are specifically used in medical diagnostics and treatment, industrial radiography involves the use of radiation to inspect materials and structures, and nuclear power plants emit radiation as a byproduct of the nuclear fission process used to generate electricity. These artificial sources are not considered natural because they are the result of human activity and technology.