Terminal "R" on a thermostat is used for ____________?

The correct function of terminal "R" on a thermostat is to provide 24-V AC input. This terminal connects to the transformer that supplies the necessary voltage to the thermostat, allowing it to receive power and operate effectively.

This 24-V AC input is essential because it enables the thermostat to control heating and cooling systems by sending signals to the equipment based on the temperature settings and sensors within the home. The "R" terminal is typically associated with the power supply side of the thermostat and can be thought of as the "hot" side of the transformer circuit.

As for the other options, they are defined functions associated with other terminals. Heat input, cooling input, and common input typically refer to other specific connections required for functional operation of the HVAC system but do not directly pertain to the primary power source that terminal "R" represents. Understanding this distinction is crucial for correctly wiring and troubleshooting thermostats within HVAC systems.