Delay period in a CI engine refers to the time interval between the start of fuel injection and the start of combustion. It is an important parameter as it affects the overall combustion process and engine performance. The delay period is influenced by various factors, and the correct answer is option 'D', which states that it depends on temperature and pressure in the cylinder at the time of injection, the nature of the fuel mixture strength, and the relative velocity between the fuel injection and air turbulence pressure of residual gases. Let's understand each of these factors in detail:

Temperature and pressure in the cylinder at the time of injection:
- The delay period is affected by the initial temperature and pressure conditions inside the cylinder. Higher temperatures and pressures promote faster combustion and reduce the delay period.
- When the temperature and pressure in the cylinder are low, the fuel takes more time to vaporize and mix with air, resulting in a longer delay period.
- The temperature and pressure are influenced by various engine parameters such as engine speed, load, compression ratio, and fuel injection timing.

Nature of the fuel mixture strength:
- The fuel mixture strength refers to the ratio of fuel mass to the air mass in the combustion chamber. It determines the richness or leaness of the mixture.
- A rich mixture with higher fuel concentration requires more time to mix with air, resulting in a longer delay period.
- On the other hand, a lean mixture with lower fuel concentration combusts more quickly, leading to a shorter delay period.
- The fuel mixture strength is controlled by the fuel injection system and can be adjusted based on the engine operating conditions.

Relative velocity between the fuel injection and air turbulence pressure of residual gases:
- The delay period is also influenced by the relative velocity between the fuel injection and the air turbulence pressure of residual gases present in the cylinder.
- Higher relative velocity promotes better mixing of fuel and air, reducing the delay period.
- The air turbulence pressure of residual gases helps in breaking up the injected fuel into smaller droplets, facilitating faster vaporization and mixing with air.

In conclusion, the delay period in a CI engine depends on the temperature and pressure in the cylinder at the time of injection, the nature of the fuel mixture strength, and the relative velocity between the fuel injection and air turbulence pressure of residual gases. These factors collectively determine the timing and efficiency of the combustion process, affecting the overall performance of the engine.