Biological treatment plants make some hazardous-waste safe by

Biological treatment plants make some hazardous-waste safe by neutralizing wastes.

This means that now, these wastes are no longer dangerous to the surroundings and can be used for other purposes or safely disposed without harming anyone or anything. Plants obviously cannot bury anything in containers and don't have clay to cover anything with, so those answers are incorrect. They also cannot burn such high amounts of waste, so that answer is also not true.

The answer to that one is neutralizing them