What is the relationship between the AC current through a capacitor and the voltage across a capacitor?
Why A capacitor's current depends on the rate of change of voltage: i = C(dv/dt). Current is therefore largest when the voltage is changing fastest (as it crosses zero) and zero when the voltage is at its peak, which puts the current one quarter cycle, or 90 degrees, ahead of the voltage. This is the opposite of an inductor, where the voltage leads the current by 90 degrees.
Watch out The choice saying voltage leads current by 90 degrees describes an inductor, not a capacitor; in phase would describe a pure resistance.
ELI the ICE man: in a Capacitor (C), I comes before E, so current leads voltage.