The correct answer is: B. r.m.s. value.
The r.m.s. value is the effective value of an alternating current or voltage. It is the value that would produce the same heating effect in a resistor as a direct current of the same value.
The mean value of an alternating current or voltage is the average of its instantaneous values over a complete cycle. The peak value is the maximum value of an alternating current or voltage.
The average value of a sine wave is 0, while the r.m.s. value is 0.707 times the peak value. This means that a 220 V alternating current has an r.m.s. value of 155 V.
The r.m.s. value is used in calculations of power and energy, because it is a more accurate measure of the heating effect of an alternating current or voltage.