At what temperature is the coolant when the thermostat is in bypass mode?

The correct choice reflects the typical operating characteristics of an engine's cooling system. When the thermostat is in bypass mode, it means that the engine coolant has not yet reached the required temperature for the thermostat to open fully and allow coolant to flow through the radiator for cooling. Generally, the thermostat is designed to start opening at around 190°F, meaning that coolant temperatures below this threshold typically indicate that the thermostat is still bypassing coolant.

In practice, this allows the engine to warm up more efficiently without circulating coolant to the radiator, thus maintaining higher engine temperatures conducive to optimal performance and emissions control. Therefore, stating that the coolant temperature is less than 190°F aligns accurately with the operational design of thermostats used in cooling systems.