Oil circulates through the system back to the storage tank through the ____.

The oil circulates back to the storage tank through the oil return line, which is specifically designed for this purpose. The oil return line is a crucial component of the oil burner system, as it ensures that unused oil is returned to the storage tank after passing through the burner and heating process. This helps maintain an efficient and continuous flow of oil, allowing the system to operate smoothly and without interruption.

In contrast, other choices do not accurately describe this function: the filter line would typically be involved in cleaning the oil before it enters the burner; the supply line is responsible for delivering oil from the storage tank to the burner; and the vent line is used to allow air or gases to escape from the tank or system, not for returning oil. Each component has its unique role in maintaining the efficiency and safety of the oil burner operation, but the oil return line is specifically designated for the return of oil to the storage tank.