Permanent wilting point (PWP) refers to the moisture content in the soil at which plants are no longer able to extract water effectively, leading to permanent wilting and eventually death. PWP is an important parameter in agriculture and irrigation planning, as it helps determine the irrigation needs of crops.

The correct answer to the question is option 'A', which represents hygroscopic water. Hygroscopic water is the moisture content in the soil that is held tightly by the soil particles and is unavailable to plants. It is the water that remains in the soil even at low water potentials. This water is strongly attracted to the soil particles and requires significant energy for plants to extract it.

Here is a detailed explanation of why option 'A' is the correct answer:

1. Hygroscopic Water:
- Hygroscopic water is the moisture content in the soil that is held tightly by the soil particles.
- It is the water that remains in the soil even at low water potentials.
- This water is strongly attracted to the soil particles and requires significant energy for plants to extract it.
- It is not readily available to plants and does not contribute to the plant's water needs.
- At the permanent wilting point, the remaining moisture in the soil is primarily hygroscopic water.

2. Field Capacity Water:
- Field capacity water refers to the maximum amount of water that can be held in the soil after excess water has drained away.
- It represents the water content at which the soil is saturated but not waterlogged.
- It is readily available to plants and can be used by the roots for their water needs.

3. Capillary Water:
- Capillary water refers to the water held in the soil against the force of gravity.
- It is the water that is held in the soil's small pores and can be easily extracted by plant roots.
- It is readily available to plants and contributes to their water needs.

4. Water of Adhesion:
- Water of adhesion refers to the water that is held in the soil due to the attractive forces between water molecules and soil particles.
- It is not readily available to plants and does not contribute significantly to their water needs.

In conclusion, the correct answer to the question is option 'A', representing hygroscopic water. Permanent wilting point moisture content for a crop represents the moisture content in the soil that is tightly held by the soil particles and is unavailable to plants.