An AC voltmeter is calibrated to read the:

This question tests the same practical concept as general AC voltmeter calibration, but relies on a different vocabulary term. In electrical engineering, 'effective value' is simply another name for the 'RMS value'. Because we use AC voltage to do real work (like lighting a bulb or heating an element), standard multimeters and AC voltmeters are calibrated to show this effective value. It tells the operator how much DC voltage would be required to achieve the exact same effect. Measurements like peak or peak-to-peak are certainly useful, but they are typically read using an oscilloscope rather than a standard voltmeter.