What device is used to store an electric charge?

A capacitor is the correct device used to store an electric charge. It consists of two conductive plates separated by an insulating material known as a dielectric. When a voltage is applied across the plates, an electric field is created, allowing the capacitor to accumulate charge. This stored charge can then be released when needed, making capacitors essential components in circuits for smoothing out voltage fluctuations, timing applications, and energy storage systems.

In contrast, an electroscope is used primarily to detect the presence of electric charge but does not store it. A magnet primarily interacts with magnetic fields rather than electric charge storage and doesn't have a method to hold electric charge. A transformer serves to transfer electrical energy between circuits through electromagnetic induction, transforming voltage levels but not storing electric charge itself.