For the destruction of CDSA substances, which of the following must be done?

The appropriate method for the destruction of Controlled Drug and Substances Act (CDSA) substances involves mixing them with a soap-water slurry. This option ensures that the substances are rendered unusable, which is a critical step in preventing accidental ingestion or misuse. When mixed with an inert substance like soap and water, the resulting mixture is less likely to be removed and misused while also facilitating a safer disposal method.

This approach aligns with established best practices for hazardous waste disposal, which emphasize making controlled substances unrecognizable and unpalatable to further discourage improper access. The other methods presented do not adequately ensure safety and regulatory compliance. For instance, flushing drugs down the drain can contaminate water supplies and harm the environment. Similarly, burying substances underground may not ensure they are adequately secured from retrieval, and rendering them useful would completely contradict the principles of safe disposal.