What characteristic best identifies a natural source of radiation?

A natural source of radiation is characterized by its ability to spontaneously emit excess energy. This emission occurs without the need for external stimulation or excitation. Natural sources, such as uranium, radon, and other radioactive materials, release radiation as part of their decay process, which is intrinsic to their properties. This process can be continuous and is not dependent on other energies being present.

The other options describe characteristics that do not define natural radiation sources. Emitting energy when excited by x-rays relates to induced radiation, which is not a characteristic of natural radiation. Avoiding decay is inaccurate since all natural radioactive sources will eventually decay, albeit at different rates. Glowing in the dark may refer to certain materials exhibiting fluorescence or phosphorescence, but this is not a hallmark of natural radiation sources; rather, it can occur in various substances regardless of their radiological properties.