What type of fuel is primarily processed for use as biodiesel?

Biodiesel is primarily produced from renewable sources, with animal fats being one of the key feedstocks. The process involves a chemical reaction called transesterification, where animal fats or vegetable oils are converted into biodiesel by reacting with an alcohol, usually methanol, in the presence of a catalyst. This results in fatty acid methyl esters (FAME), which make up biodiesel.

Animal fats provide a substantial amount of triglycerides, which are essential for the biodiesel production process. In addition to animal fats, vegetable oils are also widely used, but animal fats serve as a critical component, especially given their availability and lower cost in some regions.

Other options listed, such as heavy metals, natural gas, and propane, do not pertain to biodiesel production. Heavy metals are not a fuel source; natural gas is primarily used for gaseous fuels and not for biodiesel; and propane is a byproduct of natural gas processing and petroleum refining, used mainly as a vehicle fuel but not in biodiesel synthesis. Therefore, the focus on animal fats as the primary processing source for biodiesel makes this the correct choice.