The correct answer is B) Root-mean-square value. Ammeters and voltmeters used in sinusoidal AC power systems indicate the root-mean-square (RMS) value of the waveform. The RMS value represents the effective or equivalent DC value that would produce the same power as the AC waveform. This is the standard measurement used for AC systems, as it provides a meaningful representation of the power being delivered, regardless of the waveform shape. The other options are incorrect because they do not accurately represent the value indicated by standard AC measuring instruments. The maximum value (A) and peak value (C) are higher than the RMS value, while the average value (D) is lower than the RMS value for a sinusoidal waveform.