The knocking tendency in compression ignition engines refers to the undesirable phenomenon of uncontrolled combustion that occurs ahead of the normal flame front. It produces a knocking or pinging sound and can lead to engine damage if not addressed. The knocking tendency is influenced by various factors, and in this case, the correct answer is option 'C' - the decrease of compression ratio.

A compression ignition engine operates by compressing the air-fuel mixture to a high pressure and temperature before igniting it with the help of a spark plug. The compression ratio is the ratio of the maximum volume to the minimum volume in the combustion chamber. It plays a crucial role in the efficiency and performance of the engine.

Here's why the knocking tendency increases with a decrease in the compression ratio:

1. Understanding compression ratio:
- The compression ratio determines the pressure and temperature achieved during the compression stroke.
- A higher compression ratio leads to higher pressures and temperatures, which help in efficient combustion and power generation.
- On the other hand, a lower compression ratio results in lower pressures and temperatures.

2. Effect of lower compression ratio on knocking tendency:
- When the compression ratio is reduced, the air-fuel mixture is not compressed as much as before.
- As a result, the temperature and pressure attained during compression are lower.
- Lower temperatures can lead to incomplete combustion, which increases the chances of knocking.
- The reduced pressure also affects the ignition delay, which is the time between fuel injection and ignition. A longer ignition delay can contribute to knocking.

3. Other factors affecting knocking tendency:
- While the compression ratio is the most significant factor in this case, other factors can also influence the knocking tendency.
- Higher coolant water temperature (option 'A') can actually help reduce knocking as it helps in cooling the combustion chamber and reducing the temperature.
- The temperature of the inlet air (option 'B') can affect the combustion process, but its direct impact on knocking tendency is not as significant as the compression ratio.
- Higher compression ratio (option 'D') can increase the knocking tendency if it leads to excessively high temperatures and pressures, but a decrease in compression ratio is more likely to cause knocking.

In conclusion, the knocking tendency in compression ignition engines increases with a decrease in the compression ratio. This is because a lower compression ratio results in lower temperatures and pressures during the compression stroke, leading to incomplete combustion and increased chances of knocking. Other factors such as coolant water temperature and inlet air temperature can also influence knocking, but their impact is not as significant as the compression ratio.