Before disposal as regular waste, radioactive waste must be ________ to background radiation levels.

The chosen answer is correct because radioactive waste must be decayed to background radiation levels before it can be safely disposed of as regular waste. The process of decay refers to the natural reduction of radioactive isotopes over time, which diminishes their level of radioactivity. Once the radiation levels return to that of the surrounding environment—often referred to as background radiation levels—this waste can be treated as regular waste without posing a risk to human health or the environment.

This process often involves storage for a specific duration, allowing for the isotopes to decay sufficiently. While the other options may relate to aspects of waste management—like transportation or incineration—they do not address the fundamental requirement of reducing radioactivity through natural decay. Therefore, ensuring that radioactive waste is allowed to decay is crucial for safe disposal practices.