Which type of heater is installed internally in the engine block, in place of a freeze plug?

The immersion heater is the correct choice because it is specifically designed to be installed directly inside the engine block, replacing a freeze plug. This setup allows the heater to heat the engine coolant effectively, promoting quicker engine warm-up times, particularly in cold weather conditions. An immersion heater directly contacts the coolant, ensuring uniform heating throughout the system.

In contrast, circulating heaters function by heating the coolant that then circulates throughout the engine via the vehicle's existing coolant system, but they are not installed within the engine block. Radiant heaters, meanwhile, typically work by radiating heat onto the surrounding area rather than transferring heat directly to the engine components. Thermal heaters might refer to various heating methods that do not specifically meet the criteria of being an internal heating device within the engine block. Thus, immersion heaters stand out as the ideal choice for this application due to their direct integration and efficiency in heating engine components.