The correct answer is A. True power.
True power is the power that is actually used to do work in a circuit. It is measured in watts (W). It is equal to the product of the voltage and current, multiplied by the cosine of the phase angle between them.
Apparent power is the total power that is delivered to a circuit. It is measured in volt-amperes (VA). It is equal to the product of the voltage and current.
Reactive power is the power that is stored in the reactive components of a circuit, such as inductors and capacitors. It is measured in volt-amperes reactive (VAR).
Instantaneous power is the power that is flowing in a circuit at a given instant in time. It is equal to the product of the voltage and current at that instant.
The product of apparent power and cosine of the phase angle between circuit voltage and current is true power. This is because the cosine of the phase angle is a measure of how much of the apparent power is actually being used to do work.