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

Natural background radiation is primarily composed of radiation that occurs naturally in the environment and is not influenced by human activities. Cosmic rays are a significant component of this natural radiation, originating from outer space and interacting with the Earth's atmosphere. These high-energy particles can penetrate the atmosphere and reach the surface, contributing to the overall exposure to radiation that individuals experience.

In contrast, medical imaging, nuclear power plants, and industrial radiation sources are all tied to human-made or anthropogenic sources of radiation. Medical imaging involves procedures such as X-rays or CT scans, which introduce artificial radiation exposure. Nuclear power plants emit radiation as part of their operation and waste management but do not contribute to the natural background radiation. Similarly, industrial radiation sources are utilized for various applications and are not naturally occurring.

Thus, the correct answer highlights cosmic rays as a fundamental source of natural background radiation that affects all living organisms and the environment continuously.