The correct answer is option 'C' - low frequency. Let's understand why small capacitors are open circuited in the low frequency region.

Introduction to Capacitors:
A capacitor is an electrical component that stores and releases electrical energy. It consists of two conductive plates separated by an insulating material called a dielectric. When a voltage is applied across the plates, an electric field is established, and the capacitor stores energy in the form of electric charge.

Behavior of Capacitors at Different Frequencies:
The behavior of capacitors depends on the frequency of the applied voltage. At different frequencies, capacitors can exhibit different characteristics.

1. High Frequency:
At high frequencies, the impedance of a capacitor decreases. This means that capacitors become more conductive and allow the flow of electric current. Small capacitors, typically used in high-frequency applications, are designed to have low impedance even at high frequencies. Therefore, small capacitors are not open circuited in the high-frequency region.

2. Medium Frequency:
At medium frequencies, the impedance of a capacitor is in the intermediate range. Capacitors can still allow the flow of electric current, but their impedance is higher compared to high frequencies. Small capacitors can still function effectively in the medium-frequency region.

3. Low Frequency:
At low frequencies, the impedance of a capacitor increases significantly. This means that capacitors become less conductive and restrict the flow of electric current. In the low-frequency region, the impedance of small capacitors can become so high that they effectively act as open circuits. This is why small capacitors are open circuited in the low-frequency region.

Reason for Open Circuit:
The reason for the open circuit behavior of small capacitors in the low-frequency region is the capacitive reactance. Capacitive reactance is the opposition to the flow of alternating current (AC) through a capacitor. It is inversely proportional to the frequency of the applied voltage.

At low frequencies, the capacitive reactance becomes large, limiting the current flow through the capacitor. This effectively makes the capacitor behave as an open circuit because only a negligible current can pass through it. As a result, the voltage across the capacitor remains constant, and no significant energy is stored or released.

Conclusion:
In summary, small capacitors are open circuited in the low-frequency region due to their high impedance or capacitive reactance. At low frequencies, the impedance of capacitors increases, restricting the flow of electric current and rendering small capacitors ineffective for storing or releasing electrical energy.

Capacitive reactance is inversely proportional to frequency so at low frequency smaller capacitors will offer very high impedance so it will act as an open circuit. hence effect of such small capacitance in shunt can be neglected