Which of the following is NOT a natural source of radiation?

• A. Radon
• B. Cosmic Rays
• C. Terrestrial sources
• D. Medical X-rays

Answer: (D)

The question tests the difference between natural background radiation and human-made sources. Natural sources are part of the environment: radon gas from soil and rocks, cosmic rays from space that reach Earth, and terrestrial radiation from naturally occurring radionuclides in rocks and building materials. These exist without deliberate human activity and contribute to the background dose people receive daily. Medical X-rays, however, are produced by X-ray machines through human-made equipment and are not naturally present in the environment. They are designed for diagnostic imaging, so they are considered artificial sources of radiation. Therefore, the option that is not a natural source of radiation is the medical X-ray exposure.