Seal coat is provided to:

a) Serve as a thin wearing course: A seal coat is a thin layer of asphaltic material that is applied to the surface of a pavement. It serves as a wearing course, which means it provides a smooth and skid-resistant surface for vehicles to travel on. The seal coat acts as a protective layer that helps to reduce the effects of traffic loads and weathering on the underlying pavement structure.

b) Protect the base course: The base course of a pavement is the layer that provides support to the surface layer and distributes the load from traffic. The seal coat helps to protect the base course by acting as a barrier against moisture, preventing it from seeping into the pavement layers. Moisture can weaken the base course and lead to structural damage, so the seal coat helps to extend the lifespan of the pavement by keeping it dry.

c) Waterproof the pavement surface: One of the main functions of a seal coat is to waterproof the pavement surface. Water is one of the biggest threats to the durability of a pavement. It can penetrate the surface and cause damage to the underlying layers, leading to cracks, potholes, and other forms of deterioration. The seal coat forms a waterproof barrier that prevents water from infiltrating the pavement, thus preserving its integrity and reducing the need for costly repairs.

d) Do all the above: The correct answer is option 'd' because a seal coat serves all the mentioned purposes. It acts as a thin wearing course, protects the base course, and waterproofs the pavement surface. By providing a smooth and skid-resistant surface, protecting the base course, and preventing water infiltration, a seal coat helps to enhance the overall performance and longevity of the pavement.

In conclusion, a seal coat is a crucial component in pavement construction and maintenance. Its primary function is to serve as a thin wearing course, protect the base course, and waterproof the pavement surface. By fulfilling these functions, a seal coat helps to ensure the durability and longevity of the pavement, reducing the need for frequent repairs and improving the overall quality of the road network.