What alerts the operator in case of excessive smoke or improper combustion?

The correct answer, which is the smoke alarm, serves as a critical safety device for an oil burner operator. A smoke alarm is designed to detect smoke particles that indicate improper combustion or burning inefficiencies. When fuel does not combust completely, it often generates excess smoke, which the smoke alarm can sense. As a safety measure, the smoke alarm alerts the operator to take immediate action, preventing dangerous situations like fire or the release of harmful gases.

The other devices listed serve different purposes. The ignition switch is used to start the burner but does not monitor combustion. A heat detector would alert to unusually high temperatures but may not indicate smoke or improper combustion directly. Similarly, a flow switch monitors the flow of oil to the burner but does not provide feedback on combustion quality. Thus, the smoke alarm is the most relevant and effective tool for alerting the operator about excessive smoke or combustion issues.