The correct answer is C) stenching. The process of adding mercaptans (sulfur-containing compounds) to natural gas or propane is known as "stenching" or "odorizing." This is done to give the otherwise odorless fuel a distinct, unpleasant smell. This allows users to detect even small gas leaks, which is an important safety precaution. The other options, "reeking," "smelling," and "saturating," do not accurately describe this specific process of intentionally adding odorants to fuel gases.