Final answer:
The primary voltage of a residential furnace transformer is typically 120 volts AC, 1 phase, and 60 Hertz, making the correct answer D. 120/1/60. Secondary voltage for control circuits is usually 24 volts AC.
Step-by-step explanation:
The primary voltage of a furnace transformer indicates the voltage applied to its primary coil. In most residential settings, the standard voltage for furnaces is 120 volts AC with a single phase and a 60 Hz frequency. Therefore, the correct option would be D. 120/1/60, which denotes 120 volts, 1 phase, and 60 Hertz. This is representative of typical North American residential electricity supplies. It is worth mentioning that the secondary voltage often used for the control circuits within the furnace system is typically around 24 volts AC, which is transformed down from the 120-volt primary voltage.