What voltage is found from terminal R to terminal G on the thermostat?

The correct voltage found from terminal R to terminal G on the thermostat is 24 volts. This is standard in most HVAC systems that use a low-voltage control circuit. The R terminal is typically designated for the power supply (often connected to a 24-volt transformer), and the G terminal is connected to the fan. When the thermostat calls for cooling or heating, it sends a signal from R to G, activating the system's fan.

This low-voltage operation is crucial for safety and efficiency, as it allows the thermostat to control HVAC equipment without requiring high-voltage wiring. The other voltage options listed—12 volts, 120 volts, and 240 volts—are either too low or represent line voltages used in different parts of electrical systems, not in the low-voltage control circuits of thermostats.