The correct answer is: A. 180°
When the phase difference between the current and voltage is 180°, the current and voltage are in opposite phase. This means that the current is at its maximum when the voltage is at its minimum, and vice versa. When this happens, the power consumed by the circuit element is zero.
To understand this, let’s consider a simple circuit with a resistor. The power consumed by the resistor is given by the equation $P = I^2R$, where $I$ is the current through the resistor and $R$ is the resistance of the resistor.
If the current and voltage are in phase, then the power consumed by the resistor is maximum. This is because the current and voltage are both positive at the same time, so they are both contributing to the power consumption.
However, if the current and voltage are in opposite phase, then the power consumed by the resistor is zero. This is because the current is positive when the voltage is negative, and vice versa. So, the current and voltage are cancelling each other out, and there is no power being consumed.
In general, the power consumed by a circuit element will be least when the phase difference between the current and voltage is 180°.