The correct answer is option 'A' - an open circuit.

Explanation:
A capacitor is an electrical component that stores and releases electrical energy. It consists of two conductive plates separated by an insulating material known as a dielectric. When a DC (direct current) voltage is applied across the capacitor, it charges up and stores energy in the form of an electric field between its plates.

In a DC circuit, the behavior of a capacitor can be explained by its charging and discharging characteristics. Let's discuss how a capacitor acts as an open circuit in a DC circuit.

Charging Phase:
- Initially, when a DC voltage is applied across a discharged capacitor, it behaves as an open circuit. This means that no current flows through the capacitor, and it blocks the flow of DC current through it.
- As the voltage is applied, the electric field starts building up between the capacitor plates. Electrons accumulate on one plate, creating a negative charge, while the other plate loses electrons, creating a positive charge.
- The charging process continues until the capacitor reaches its full charge, and the voltage across it becomes equal to the applied voltage. At this point, the capacitor behaves as an open circuit, blocking any DC current from flowing through it.

Discharging Phase:
- If the applied voltage is suddenly removed from the charged capacitor, it starts to discharge. During this phase, the stored energy in the capacitor is released.
- As the capacitor discharges, the electric field between the plates collapses, and the charges on the plates redistribute.
- Similar to the charging phase, the capacitor behaves as an open circuit during the discharging process. It blocks the flow of DC current, allowing only a transient current to flow as it discharges.

Conclusion:
In a DC circuit, a capacitor acts as an open circuit during both the charging and discharging phases. It blocks the flow of DC current while allowing the flow of transient current during the charging and discharging processes. This property of a capacitor makes it useful in various applications, such as energy storage, filtering, and timing circuits.