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

Cosmic radiation is indeed a common source of background radiation. It originates from outer space and continuously bombards the Earth's atmosphere. This type of radiation is a natural and unavoidable part of our environment, contributing to the overall background radiation levels that everyone is exposed to on a daily basis.

In contrast, radiation from X-ray machines and medical diagnostic imaging are forms of artificial radiation that are typically encountered during specific medical procedures rather than as constant background exposure. Additionally, radiation from cell phones is minimal compared to cosmic radiation, as the non-ionizing radiation emitted from cell phones is much weaker and does not contribute significantly to background radiation levels. Therefore, cosmic radiation is recognized as the most relevant source of background radiation among the options presented.