Explanation:

To understand why option A is the correct answer, we need to first understand what commutation failure is and how it can occur in a circuit.

Commutation Failure:
Commutation is the process of transferring the current from one device (e.g., a thyristor) to another in a circuit. During this process, the current must be interrupted in the first device before it can be established in the second device. If the current is not interrupted properly, it can lead to commutation failure.

Thyristor Turn-Off Time:
The turn-off time of a thyristor refers to the time required for the thyristor to switch from the conducting state to the non-conducting state. During this time, the thyristor must block the reverse voltage and prevent the current from flowing through it.

Circuit Turn-Off Time:
The circuit turn-off time refers to the time required for the entire circuit to interrupt the current and switch off. It includes the turn-off time of the thyristor as well as any additional delays or factors that may affect the circuit's ability to turn off.

Importance of Circuit Turn-Off Time:
To avoid commutation failure, it is essential that the circuit turn-off time is greater than the thyristor turn-off time. This means that the circuit must be able to interrupt the current and switch off before the thyristor completes its turn-off process. If the circuit turn-off time is lesser than the thyristor turn-off time, there is a risk of the thyristor still conducting current when the circuit tries to interrupt it, leading to commutation failure.

Why Option A is Correct:
Option A states that the circuit turn-off time must be greater than the thyristor turn-off time. This is the correct answer because it ensures that the circuit has enough time to interrupt the current and switch off before the thyristor completes its turn-off process. By allowing a sufficient margin between the circuit turn-off time and the thyristor turn-off time, we can avoid commutation failure and ensure proper operation of the circuit.

Conclusion:
To avoid commutation failure, it is crucial to have a circuit turn-off time that is greater than the thyristor turn-off time. This allows the circuit to interrupt the current and switch off before the thyristor completes its turn-off process. Option A accurately describes this requirement and is therefore the correct answer.