The thermostat in the engine cooling system is turned on by?

The correct answer is based on the function of a thermostat in an engine cooling system. The thermostat is a temperature-controlled device that regulates the flow of coolant to maintain the engine at an optimal operating temperature. When the engine is cold, the thermostat remains closed, preventing coolant from flowing to the radiator. As the engine reaches the designated operating temperature, the cooling water temperature rises, causing the thermostat to open and allow coolant to circulate through the engine and radiator. This action helps prevent the engine from overheating and ensures that it operates efficiently.

The other options relate to different factors that do not directly control the operation of the thermostat. Coolant pressure, engine RPM, and ambient temperature can impact engine performance but do not directly trigger the thermostat's opening or closing mechanism. The thermostat specifically responds to the temperature of the cooling water, making this option the most accurate choice for how the thermostat in the engine cooling system is activated.