Increasing the tendency for knocking in a Cl engine refers to the phenomenon where the air-fuel mixture ignites prematurely or unevenly in the combustion chamber, resulting in a knocking sound. Knocking can cause damage to the engine and decrease its efficiency. Several factors contribute to the likelihood of knocking, and among the given options, option 'C' is the correct choice. Let's delve into the details of each factor:

1. Speed: Increasing the engine speed can lead to knocking because the air-fuel mixture has less time to fully burn before the next combustion cycle begins. This increases the chances of incomplete combustion and knocking.

2. Injection advance: Injection advance refers to the timing at which the fuel is injected into the combustion chamber. Advancing the injection timing means injecting the fuel earlier in the compression stroke. When the injection advance is increased, the fuel has more time to vaporize and mix with the air in the cylinder. However, if the injection advance is too high, it can cause the mixture to ignite prematurely, leading to knocking.

3. Coolant temperature: The coolant temperature affects the engine's operating temperature. When the coolant temperature is increased, it can lead to higher cylinder temperatures. This can cause the air-fuel mixture to ignite prematurely and result in knocking.

Therefore, increasing the speed, injection advance, and coolant temperature (option 'C') all contribute to an increased tendency for knocking in a Cl engine. These factors can create conditions where the air-fuel mixture is more prone to premature combustion or uneven burning, leading to the knocking phenomenon.

On the other hand, increasing the compression ratio (option 'D') alone does not directly increase the tendency for knocking. A higher compression ratio can actually improve engine efficiency and power output. However, if the compression ratio is excessively high, it can lead to increased cylinder pressures and temperatures, which may result in knocking if other factors are also present.

In summary, the tendency for knocking in a Cl engine is increased by a combination of factors including speed, injection advance, and coolant temperature. These factors can create conditions where the air-fuel mixture is more likely to ignite prematurely or unevenly, resulting in knocking.