Introduction:
Knocking is an undesirable phenomenon that occurs in internal combustion engines, particularly in spark-ignition engines. It is characterized by a knocking or pinging sound, which is caused by the air-fuel mixture in the combustion chamber detonating prematurely. Knocking leads to reduced engine performance, increased fuel consumption, and can potentially cause engine damage.

Explanation:
The knocking tendency in a spark-ignition engine can be influenced by several factors. In this case, we are specifically looking at the factors that reduce the knocking tendency.

1. Compression Ratio:
The compression ratio is the ratio of the cylinder volume at the bottom dead center (BDC) to the cylinder volume at the top dead center (TDC). A higher compression ratio means that the air-fuel mixture is compressed to a smaller volume before ignition, leading to a higher temperature and pressure.

- As the compression ratio increases, the knocking tendency in the engine decreases. This is because a higher compression ratio promotes better combustion efficiency, reducing the chances of premature detonation.

2. Wall Temperature:
The wall temperature refers to the temperature of the cylinder walls in the combustion chamber. It can be influenced by factors such as cooling system efficiency and heat transfer characteristics of the engine components.

- Increasing the wall temperature can actually increase the knocking tendency in the engine. This is because higher wall temperatures can lead to localized hot spots in the combustion chamber, which can initiate and propagate the knocking phenomenon.

3. Supercharging:
Supercharging is the process of increasing the air pressure and density in the intake manifold of an engine. This is typically achieved by using a mechanical or exhaust-driven device called a supercharger.

- Supercharging can reduce the knocking tendency in an engine. By increasing the air density in the combustion chamber, supercharging helps to cool the intake charge, making it less prone to knock.

4. Engine Speed:
Engine speed, also known as RPM (revolutions per minute), refers to the number of crankshaft rotations per unit of time. It is a measure of how fast the engine is running.

- The knocking tendency in an engine reduces with increasing engine speed. This is because at higher engine speeds, the air-fuel mixture has less time to react and pre-ignite before the spark plug ignites it. Therefore, the chances of knocking decrease at higher engine speeds.

Conclusion:
In summary, the knocking tendency in a spark-ignition engine is reduced with increasing engine speed. This is due to the reduced reaction time for the air-fuel mixture to pre-ignite before the spark plug ignites it. The other factors mentioned, such as compression ratio, wall temperature, and supercharging, can also have an impact on the knocking tendency, but in this case, the correct answer is engine speed.