The correct option is A Capacitor
A device used to store an electric charge, consisting of one or more pairs of conductors separated by an insulator is known as capacitor. A single conductor can store small charge. When we give charge to it, its potential reaches to maximum value quickly, now it can't store more charge. If we place another conductor near it, its potential becomes low and it is able to store more charge, this is the principle of a capacitor.