**Under Commutation**

Under commutation refers to a situation where the commutation time is longer than the critical commutation time. In this case, the practical commutation time observed is 2.5 ms, which is greater than the critical commutation time of 2 ms. Therefore, the correct answer is option 'A' - under commutation.

**Explanation:**

1. Commutation is the process of reversing the current in a conducting device, such as a diode or a thyristor, to maintain a unidirectional flow of current in a circuit.

2. During commutation, there is a brief period when both the outgoing and incoming devices are conducting simultaneously. This period is known as the commutation overlap.

3. The duration of the commutation overlap is critical for the proper operation of the circuit. If the overlap is too short, it can result in a high rate of change of current (di/dt) and voltage (dv/dt), leading to excessive stress on the devices and possible malfunction.

4. The critical commutation time refers to the maximum allowable duration of the commutation overlap to ensure proper device operation and minimize stress.

5. In this case, the critical commutation time is specified as 2 ms, which means that the commutation overlap should not exceed 2 ms.

6. However, it is observed that the practical commutation took 2.5 ms, which is longer than the critical commutation time of 2 ms. This indicates that the commutation overlap duration has exceeded the allowable limit.

7. When the commutation overlap duration is longer than the critical commutation time, it results in under commutation. This can lead to various issues such as increased switching losses, higher stress on the devices, and potential circuit malfunction.

8. Therefore, the correct answer is option 'A' - under commutation, as the observed practical commutation time of 2.5 ms exceeds the critical commutation time of 2 ms.