What are natural sources of radiation?

Natural sources of radiation include cosmic rays, terrestrial radiation, and radionuclides, making this answer the most accurate choice.

Cosmic rays originate from outer space and interact with the Earth's atmosphere, emitting a form of radiation that humans are exposed to continuously. Terrestrial radiation comes from radioactive materials present in the Earth's crust, such as uranium, thorium, and radon. Radionuclides, which are unstable isotopes found in the environment, also contribute to the natural background radiation we receive from soil, rocks, and even from within our own bodies.

In contrast, other options refer primarily to artificial sources. Medical x-rays and consumer products introduce man-made sources of radiation exposure, while radioactive waste and nuclear power plants are byproducts of human activities in nuclear energy generation. Similarly, manufactured isotopes and smoke detectors involve human-engineered elements designed for specific uses in safety and medicine, differing fundamentally from natural radiation sources.