Introduction:
A wet sieve analysis is a type of particle size analysis that is carried out when the sample is in the form of a very fine powdered material. This method involves passing the sample through a series of sieves while adding water to facilitate the separation of particles based on their size.

Explanation:
The correct answer is option D, which states that a wet sieve analysis is carried out when the sample is a very fine powdered material. Here's why:

1. Purpose of a sieve analysis:
A sieve analysis is performed to determine the particle size distribution of a sample. It helps in understanding the physical properties and behavior of the material. By analyzing the particle size distribution, engineers and researchers can make informed decisions regarding the suitability of the material for various applications.

2. Limitations of dry sieve analysis:
In a dry sieve analysis, the sample is sieved without adding any water or other liquid. This method is suitable for coarse-grained materials, but it becomes ineffective for very fine powders. The particles in a fine powder tend to stick together, resulting in clogging of the sieves and inaccurate results. Therefore, a wet sieve analysis is preferred in such cases.

3. Principle of wet sieve analysis:
In a wet sieve analysis, water is added to the powdered sample to create a slurry. The slurry is then poured onto a stack of sieves arranged in decreasing order of mesh size. The water helps in breaking the particle agglomerates and allows the particles to pass through the sieves more easily.

4. Advantages of wet sieve analysis:
- Improved accuracy: The addition of water helps in separating the fine particles and prevents clogging of the sieves, resulting in more accurate results.
- Better dispersion: The slurry created by adding water ensures better dispersion of the particles, leading to a more representative sample for analysis.
- Reduced variability: Wet sieving reduces the variability in particle size distribution compared to dry sieving, especially for fine powders.

Conclusion:
A wet sieve analysis is carried out when the sample is a very fine powdered material. This method overcomes the limitations of dry sieving and provides more accurate results by adding water to the sample to facilitate the separation of particles based on their size. Wet sieve analysis is widely used in various industries, including civil engineering, to determine the particle size distribution of materials.