Which of the following is a source of radiation?

Radiation can come from various sources, and all listed options—natural background, radioactive fallout, and radiation from medical treatments—are valid sources of radiation exposure.

Natural background radiation is the radiation that is always present in the environment, originating from cosmic rays, radon gas from the ground, and other natural radioactive materials present in soil and food. This source contributes to the majority of individual radiation exposure in the general population.

Radioactive fallout refers to the residual radioactive material that descends to the ground after a nuclear explosion or accident. This material can accumulate in the environment and pose health risks depending on the levels and types of isotopes present, making it a significant source of exposure, particularly in affected areas.

Radiation from medical treatments includes diagnostic procedures like X-rays, CT scans, and therapeutic applications such as radiation therapy for cancer. These medical interventions are beneficial for diagnosis and treatment but do contribute to a person's overall radiation exposure.

Since all these sources contribute to the total radiation exposure experienced by individuals in various contexts, the answer indicating that all of them are sources of radiation is indeed the most comprehensive choice.