What characteristic of a material may increase its radiological hazard?

The characteristic that significantly increases the radiological hazard of a material is the presence of highly radioactive isotopes. Highly radioactive isotopes emit radiation that can be harmful to living organisms, and their decay can release energy in the form of alpha, beta, or gamma radiation. The intensity and type of radiation would dictate the level of hazard associated with exposure, as different isotopes can have varying half-lives and biological effects.

In contrast, the absence of an odor does not relate to the radiological hazard a material poses, as many radioactive materials may not have any scent, but still be extremely hazardous. Similarly, a material's capability to conduct electricity and its density do not inherently dictate its radiological characteristics. While density could correlate with the material's atomic structure, it does not serve as a direct measure of radiological risk. Thus, the clear link between highly radioactive isotopes and increased risk is the reason why this choice is correct.