Why are unborn children considered at the greatest risk regarding radiation exposure?

Unborn children are considered at the greatest risk regarding radiation exposure primarily because they are growing rapidly and their cells are more sensitive to the effects of radiation. During the early stages of development, the rapid division of cells means that any damaging effects of radiation, such as alterations to DNA, can lead to serious consequences. This susceptibility is heightened because the fetal tissues that are developing are still differentiating into specialized cells and systems, which makes them more vulnerable to the harmful effects of ionizing radiation.

Additionally, the concept of the "health risk window" is critical; certain stages of development may be more sensitive to radiation exposure, particularly during the first trimester when the foundations of major organs are being formed. This increased sensitivity can lead to various complications, including growth issues, developmental abnormalities, and increased risk of certain cancers later in life.

In contrast, the other options do not accurately convey the reasons for the heightened risk. For example, unborn children are not more resilient to radiation, nor are they less likely to be exposed. They absorb radiation, and the degree of risk is significant because of their rapid growth and developmental processes.