Final answer:
Background radiation is a type of ionizing radiation that is present in the environment from natural and human-made sources. It contributes to about half of the average American's total exposure. While it is a potential risk factor for cancer, it is not the sole cause.
Step-by-step explanation:
Background radiation is a type of ionizing radiation that is present in the environment from natural and human-made sources. It includes exposure to x-rays, nuclear medicine isotopes, cosmic radiation, and radon gas. According to the given information, background radiation contributes to about half of the average American's total exposure. However, the specific percentage mentioned in the options (a. 20% b. 30% c. 40% d. 50%) is not provided, so it is difficult to determine the exact contribution of background radiation to fatal cancer. It is important to note that while background radiation is a potential risk factor for cancer, it is not the sole cause, as there are many other factors involved in the development of cancer.