An AC source is connected to a diode and a resistor in series. Is the current through the resistor AC or DC?
When an AC source is connected to a diode half rectification of the source will happen and we will get a pulsating DC output.
The resistor in series would be getting a pulsating DC output. The given circuit in question is analogous to half rectification of AC source by diode, in that circuit we use resistor for measuring the load of DC.