The correct answer is A. Metallic plate variable gang condenser.
A metallic plate variable gang condenser is a type of capacitor that is suitable for D.C. only. It is made up of two plates of metal that are separated by a dielectric material. The dielectric material can be air, paper, or plastic. The plates are connected to a variable resistor, which allows the user to change the capacitance of the capacitor.
Metallic paper capacitor is a type of capacitor that is made up of two metal plates that are separated by a layer of paper. The paper is impregnated with an insulating material, such as oil or wax. Metallic paper capacitors are used in a variety of applications, including power supplies, filters, and oscillators.
Oil impregnated paper condenser is a type of capacitor that is made up of two metal plates that are separated by a layer of paper that is impregnated with oil. The oil acts as an insulator and helps to prevent the plates from touching. Oil impregnated paper condensers are used in a variety of applications, including power supplies, filters, and oscillators.
Poled aluminium electrolytic condenser is a type of capacitor that is made up of two aluminum plates that are separated by an electrolyte. The electrolyte is a liquid that conducts electricity. Poled aluminum electrolytic capacitors are used in a variety of applications, including power supplies, filters, and oscillators.
However, poled aluminum electrolytic capacitors are not suitable for D.C. because the electrolyte can break down when exposed to a DC voltage.