Explanation:

Flexible pavements are composed of multiple layers, including the sub-grade, base course, and surface course. The sub-grade is the layer of natural or improved soil that supports the pavement structure. It plays a crucial role in distributing the load from traffic and providing stability to the pavement.

The failure of the sub-grade can be attributed to several factors, and the given statements help identify the main causes.

1. Inadequate stability:
- The sub-grade must have sufficient stability to withstand the loads applied by traffic.
- If the sub-grade lacks stability, it can deform under the load and lead to pavement failure.
- Factors that can contribute to inadequate stability include poor soil quality, insufficient compaction, or inadequate thickness of the sub-grade layer.

2. Loss of binding action:
- The sub-grade relies on the binding action of the surrounding materials to maintain its stability.
- If the binding action is lost, the sub-grade can become weak and susceptible to deformation.
- This can occur due to the infiltration of water into the sub-grade, which causes the soil particles to separate and lose their binding properties.

3. Loss of base course materials:
- The base course is the layer between the sub-grade and the surface course, providing additional support to the pavement structure.
- While the loss of base course materials can contribute to pavement distress, it is not a direct cause of sub-grade failure.
- Loss of base course materials can lead to a reduced load-bearing capacity and increased stress on the sub-grade, but it is not the primary cause of sub-grade failure.

4. Excessive stress concentration:
- Excessive stress concentration refers to the localized areas of high stress within the pavement structure.
- These concentrated stresses can occur due to various factors, such as heavy traffic loads, inadequate pavement design, or poor construction practices.
- While excessive stress concentration can lead to pavement failure, it is not the main cause of sub-grade failure.

Conclusion:
Based on the given statements, the correct answer is option 'B' (1 and 4). The failure of the sub-grade of a flexible pavement is mainly attributed to inadequate stability and excessive stress concentration. Loss of binding action and loss of base course materials can contribute to pavement distress but are not the primary causes of sub-grade failure.