The correct answer is option 'C': grain to grain contact.

Explanation:

Load transfer in flexible pavements refers to the mechanism by which the load from vehicles is transferred from the surface layer to the underlying layers and eventually to the sub-grade. This load transfer is crucial for the overall performance and stability of the pavement structure.

Grain to grain contact plays a significant role in load transfer in flexible pavements. Here's how it works:

1. Introduction to grain to grain contact:
- In flexible pavements, the layers are typically composed of different materials such as asphalt concrete, base course, sub-base, and sub-grade.
- These materials are made up of individual particles or grains that are in contact with each other.
- The load from the vehicles is transmitted through these grains, which are in contact with each other.

2. Load transfer mechanism:
- As a vehicle passes over the pavement surface, the load is initially applied to the uppermost layer, which is the asphalt concrete layer.
- The load is distributed across the grains in the asphalt concrete layer through grain to grain contact.
- The grains in the asphalt concrete layer transmit the load to the underlying layers, such as the base course and sub-base, through grain to grain contact.
- This process continues until the load reaches the sub-grade, which is the natural soil or the compacted soil layer beneath the pavement structure.

3. Importance of grain to grain contact:
- Grain to grain contact is essential for load transfer because it allows the load to be distributed and transferred efficiently from one layer to another.
- It helps in preventing excessive deformation and stress concentration in any particular layer, which can lead to pavement failure.
- Grain to grain contact also helps in improving the overall structural integrity and stability of the pavement.

In conclusion, the load transfer in flexible pavements is primarily achieved through grain to grain contact. This mechanism allows the load from vehicles to be transmitted from the surface layer to the underlying layers, ensuring the overall performance and stability of the pavement structure.