When natural sources of ionizing radiation become increased because of accidental or deliberate human actions such as mining, they are referred to as sources.

The question is incomplete as it does not have the options which are:

artificial

enhanced natural

extraterrestrial

manmade

Enhanced natural resources

Explanation:

Natural resources are the resources used by the humans for survival present in the environment or provided by the surrounding environment.The natural resources includes the wood, gas, air, water and other substances.

The substances which emits harmful radiations like radium, thorium, uranium are known as radioactive isotopes.

When these ionizing radiation emitting natural substances increases in the environment due to human activities like the particle accelerators, X-rays and other methods which produces these substances then these substances are refereed to as the enhanced natural substances.

Thus, Enhanced natural resources is the correct answer.

not too sure but maybe in the hot springs.