What will be the reading of AC voltmeter if it is connected to a DC supply of 120V?
If you are using a scope with AC coupling, then it will read zero because it isolates DC, but if the AC coupling is off, then it will give you the true DC value.
If you are however talking about &quot;AC Voltmeters&quot; which are quite uncommon since one can easily use DC Voltmeters to read AC RMS; the ones I have dealt with use a rectifier, and holds to voltage across an RC channel and then reads the voltage as DC. The rectification can be done using a single diode or a full bridge rectifier, I found this circuit online:

Clearly this circuit will read DC values same as AC values, so long as the DC voltage is positive, if a full bridge rectifier was used instead of the single diode, then the absolute value of the voltage would be read.