What is a common source of natural background radiation?

Cosmic rays are indeed a common source of natural background radiation. These high-energy particles originate from outer space and are constantly bombarding the Earth's atmosphere. When these cosmic rays collide with particles in the atmosphere, they can create secondary particles that contribute to the radiation levels we experience on the surface of the Earth.

The natural background radiation is a result of various sources, including cosmic rays, which accounts for a significant portion of the radiation we are exposed to daily. This exposure from cosmic rays varies depending on geographic location and altitude; for instance, people living at higher altitudes receive more cosmic radiation compared to those at sea level.

The other options provided pertain to artificial sources of radiation or specific applications that do not contribute to natural background levels. X-ray machines are a source of ionizing radiation used in medical imaging but are not part of the natural environment. Similarly, radioactive waste is a byproduct of human activities, such as nuclear power generation, and does not constitute a natural source of radiation. Medical implants may contain trace amounts of radioactive material, but they are also not a source of natural background radiation.