Define "rem" in terms of radiation measurement.

The term "rem" stands for "Roentgen equivalent man," and it is used to express a unit of dose equivalent in radiation measurement. This means it quantifies the biological effect of ionizing radiation on human tissue, accounting for both the energy deposited by the radiation and its potential biological impact. The rem takes into consideration the type of radiation (such as alpha, beta, or gamma radiation) as well as its impact on different types of tissue. Thus, it is a crucial metric for evaluating health risks associated with exposure to radiation, guiding safety standards in occupational and environmental settings.

In contrast to this definition, the other options do not accurately encompass the meaning of "rem." While radiation intensity and exposure are important concepts in radiation science, they do not involve the biological effects of radiation that "rem" specifically addresses. Additionally, the classification of radioactive materials pertains to categorizing substances based on their radioactivity, which is not related to the measurement of biological dose effect that the "rem" explains.