Which of the following is classified as an underground storage tank?

An underground storage tank (UST) is defined as a tank and any underground piping connected to it that has at least 10% of its combined volume underground. According to regulations, USTs are primarily used for storing petroleum products or hazardous substances.

The waste oil tank is classified as an underground storage tank. This is because it often holds substances that could be harmful if leaked into the environment, and many waste oil tanks are situated underground to minimize risks associated with spills and contamination.

The agricultural motor vehicle fuel tank, although it may contain petroleum, is typically considered above ground if it doesn't meet the criteria of having a significant portion buried. Similarly, the residential heating oil tank, unless installed underground, generally falls outside the typical classification of USTs.

Thus, the only clear UST from the choices listed is the waste oil tank, as it fits the definition of holding potentially hazardous material underground.