What component monitors the temperature of the system?

The aquastat or temperature sensor is the correct answer because it is specifically designed to monitor the temperature of the heating system. Aquastats are crucial components in oil heating systems, as they sense the temperature of the water or the heating medium within the system. They typically work by regulating the operation of the boiler or heating system by turning it on or off based on the temperature readings they provide.

By maintaining the desired temperature, the aquastat ensures efficient operation, prevents overheating, and can help in maintaining the comfort level in the space being heated. It serves as a control mechanism that directly impacts the efficiency and safety of the heating system.

Other components, like the thermostat, are generally responsible for setting and controlling room temperature rather than monitoring the system's temperature. The pump is involved in circulating the heating fluid, and the flame sensor serves to ensure that the burner operates safely, but neither of these components directly monitors system temperature like the aquastat does.