Which of the following is a biological indicator of radiation exposure?

Blood cell counts are a critical biological indicator of radiation exposure because exposure to ionizing radiation can significantly affect the bone marrow's ability to produce blood cells. When an individual is exposed to high levels of radiation, it may lead to various changes in the blood cell counts, such as decreased red blood cells (anemia), decreased white blood cells (leukopenia), and decreased platelets (thrombocytopenia). Monitoring these changes can provide essential information about the degree of radiation exposure and potential damage to the hematopoietic system.

The other options, while they may signify changes in health status, do not serve as direct biological indicators of radiation exposure. For instance, increased immunity may suggest a functioning immune system but does not directly correlate with radiation effects. Weight gain is an outcome impacted by numerous lifestyle choices and health conditions rather than a specific response to radiation exposure. Enhanced digestion similarly does not reflect any immediate connection to radiation exposure and is influenced by many factors unrelated to radiological effects. Therefore, changes in blood cell counts remain a reliable and direct biological indicator of radiation exposure.