The correct answer is: B. r.m.s. voltage
The r.m.s. voltage is the effective voltage of an alternating current (AC) waveform. It is the value of a direct current (DC) that would produce the same heating effect in a resistor as the AC waveform.
The peak voltage is the maximum voltage of an AC waveform. It is the voltage that occurs when the waveform is at its positive or negative peak.
The average voltage of an AC waveform is the average of the positive and negative half-cycles. It is always less than the r.m.s. voltage.
In the case of a 230 V, 10 kW heater, the r.m.s. voltage is 230 V. This means that if a 10 kW resistor were connected to a 230 V DC supply, it would dissipate the same amount of power as the heater does when connected to a 230 V AC supply.
The peak voltage of the AC supply is 325 V. This is because the peak voltage of a sine wave is equal to its r.m.s. voltage multiplied by the square root of 2.