Introduction
Silicon Controlled Rectifier (SCR) is a three-terminal device that acts as a switch for controlling electrical power. It has several advantages as a switch, but one of the options provided is NOT an advantage. In this response, we will discuss each option and explain why option 'A' is not an advantage of SCR as a switch.

Advantages of SCR as a Switch

a) The switching speed is very high:
- This is an advantage of SCR as a switch. It can switch on and off very quickly, allowing for precise control of power flow.
- The high switching speed makes it suitable for applications that require fast response, such as motor control and power electronics.

b) The operation does not produce harmonics:
- This is another advantage of SCR as a switch. When it is in the on-state, it conducts current without introducing harmonics into the power system.
- This is important in applications where harmonics can cause interference or damage to other devices in the system.

c) It gives noiseless operation at high efficiency:
- This is also an advantage of SCR as a switch. It operates silently without producing any noise or vibrations.
- Additionally, SCR has a high efficiency, meaning it can handle large amounts of power with minimal losses.
- This makes it suitable for applications where noise and efficiency are important factors, such as in power supplies and industrial control systems.

d) It has no moving parts:
- This is another advantage of SCR as a switch. Being a solid-state device, it does not have any moving parts.
- The absence of moving parts makes SCR more reliable and durable compared to mechanical switches that are prone to wear and tear.
- It also eliminates the need for maintenance and reduces the risk of failure or malfunction.

Explanation for option 'A':
- The statement that "the switching speed is very high" is NOT an advantage of SCR as a switch.
- In reality, SCR has a relatively slow turn-off time compared to other semiconductor devices, such as transistors.
- This slow turn-off time can lead to higher power losses and can limit the frequency at which SCR can be switched.
- Therefore, option 'A' is the correct answer as it does not represent an advantage of SCR as a switch.

Conclusion
In conclusion, the option 'A' is not an advantage of SCR as a switch. SCR has advantages such as high efficiency, noiseless operation, absence of harmonics, and no moving parts. However, its switching speed is relatively slow compared to other semiconductor devices.