Reducing knocking in spark ignition engines is crucial for improving engine performance and preventing damage to the engine components. Knocking, also known as detonation, is an undesirable phenomenon that occurs when the air-fuel mixture in the combustion chamber ignites spontaneously and unevenly, causing a knocking sound. This can lead to reduced engine efficiency, increased fuel consumption, and potential engine damage.

The correct answer to reducing knocking in spark ignition engines is option 'C', which suggests retarding the spark advance. The spark advance refers to the timing at which the spark plug ignites the air-fuel mixture in the combustion chamber. By retarding the spark advance, the ignition timing is delayed, ensuring that the combustion process occurs at the optimal moment for efficient and controlled burning of the fuel.

Here's a detailed explanation of why retarding the spark advance helps reduce knocking in spark ignition engines:

1. Understanding Knocking:
- Knocking occurs when the air-fuel mixture in the combustion chamber ignites prematurely, before the spark plug fires.
- This can happen due to several factors, including high compression ratio, high combustion chamber temperature, and low octane rating of the fuel.
- When the unburned mixture ignites, it creates a shockwave that collides with the expanding flame front, leading to pressure waves and vibrations that cause the knocking sound.

2. Effect of Spark Advance:
- The spark advance determines when the spark plug fires in relation to the piston position.
- Advancing the spark means firing it earlier in the compression stroke, allowing more time for complete combustion.
- However, advancing the spark too much can lead to knocking if the air-fuel mixture ignites prematurely.

3. Retarding the Spark Advance:
- Retarding the spark advance means delaying the ignition timing, firing the spark plug later in the compression stroke.
- By delaying the spark, the combustion process starts when the piston is closer to the top dead center, reducing the likelihood of premature ignition.
- This helps to avoid the collision between the unburned mixture and the expanding flame front, minimizing knocking.

4. Other Methods to Reduce Knocking:
- While retarding the spark advance is an effective method, other techniques can also help reduce knocking in spark ignition engines.
- Increasing the compression ratio can improve engine efficiency, but it can also increase the likelihood of knocking.
- Increasing the cooling water temperature and the inlet air temperature can help reduce knocking by reducing the overall combustion chamber temperature.
- These methods can help cool down the air-fuel mixture, making it less prone to spontaneous ignition.

In conclusion, retarding the spark advance is an effective way to reduce knocking in spark ignition engines. By delaying the ignition timing, the likelihood of premature combustion and knocking is minimized. However, it's important to note that other factors like compression ratio, combustion chamber temperature, and fuel quality also play a significant role in knocking, and addressing them collectively can further enhance engine performance and reliability.