The correct answer is C. both D.C. as well as A.C.
An electrolytic capacitor is a type of capacitor that uses an electrolytic liquid as one of its plates. The other plate is made of a metal that reacts with the electrolyte to form a thin oxide layer. This oxide layer acts as a dielectric, which stores the electric charge.
Electrolytic capacitors can be used for both D.C. and A.C. applications. However, they are not suitable for high-frequency applications, as the oxide layer can break down at high frequencies.
Option A is incorrect because electrolytic capacitors can be used for A.C. applications.
Option B is incorrect because electrolytic capacitors can be used for D.C. applications.
Option D is incorrect because electrolytic capacitors can be used for both D.C. and A.C. applications.