What is a known effect of exposure to radiation?

The potential for radiation exposure to cause harmful biological changes is a well-documented effect, making this option accurate. When living tissues are exposed to ionizing radiation, such as that from x-rays or radioactive materials, it can lead to cellular damage, which may result in mutations, cancer, or other health issues over time. The biological effects depend on several factors, including the dose received, the duration of exposure, the type of radiation, and the biological sensitivity of the tissues involved.

In contrast to this, the other options present misconceptions about radiation safety. The idea of a threshold dose implies that there is a level of radiation exposure deemed entirely safe, but in practice, even very low doses may carry some risk. Additionally, the assertion that single large doses are safer than smaller doses contradicts the understanding that high doses can cause immediate damage, while cumulative low doses can lead to long-term health effects. Lastly, the claim that radiation from dental x-ray machines is "completely safe" overlooks the need for safety measures, appropriate usage, and the potential risk associated with any radiation exposure.