In an AC circuit containing only an ideal capacitor, the current $I(t)$ leads the voltage $V(t)$ by a phase angle of $\pi/2$ or $90^\circ$ . This eliminates options suggesting they are in phase, lead by 180°, or lag. The average power consumed over a full cycle is $P = V_{rms}I_{rms} \cos \phi$. Since the phase difference $\phi = 90^\circ$, the power factor $\cos \phi = 0$, meaning the average power consumed is zero. The capacitor stores energy during one part of the cycle and returns it to the source in the next.