Final answer:
A dielectric in a capacitor is an insulating material that, when polarized by an electric field, increases the capacitor's ability to store electrical energy.
Step-by-step explanation:
The dielectric of a capacitor is best described as an insulating material. It is neither a material that allows electric current to flow freely like a conductor, nor a substance that solely resists the flow of electric current. Instead, it is a dielectric – an electrical insulator that can be polarized by an electric field. When a dielectric is inserted between the plates of a capacitor, it increases the capacitance of the capacitor. This occurs because the dielectric becomes polarized; it induces a charge that produces an electric field opposite to that of the capacitor's plates. The presence of the dielectric reduces the electric field strength within the capacitor, allowing it to store more charge at a lower voltage, thus increasing capacitance. Therefore, the correct characterization of a dielectric in the context of a capacitor is: D) A substance that stores electrical energy.