**Explanation:**

**Camber in Hill Roads:**
Camber refers to the slight slope provided on the surface of roads to allow water to drain off easily. In hill roads, camber is particularly important as it helps prevent the accumulation of water on the road surface, which can lead to skidding and accidents.

**Thin Bituminous Surface:**
A thin bituminous surface is a type of road surface that consists of a thin layer of bituminous (asphalt) material applied on top of the road base. It is commonly used for low-volume roads or as a wearing course on high-volume roads.

**Minimum Camber Value:**
The minimum value of camber provided for thin bituminous surface hill roads is 2.5%. This means that the road surface is sloped at a rate of 2.5% from the centerline towards the edges.

**Reasoning:**
The reason for providing a minimum camber of 2.5% on thin bituminous surface hill roads is to ensure proper drainage of water. In hilly areas, water often flows down the slopes and can accumulate on the road surface, making it slippery and dangerous for vehicles.

The camber helps to create a slope on the road surface, allowing water to drain off easily to the sides. This improves the skid resistance of the road and reduces the risk of accidents, especially during rainy or snowy conditions.

**Other Options:**
- Option 'A' (2.2%): This value is lower than the correct answer of 2.5%. A lower camber value may not provide sufficient slope for water drainage, increasing the risk of accidents.
- Option 'C' (3.0%): This value is higher than the correct answer of 2.5%. While a higher camber value may provide better water drainage, it can also lead to discomfort for drivers and increase wear on the tires.
- Option 'D' (3.5%): This value is also higher than the correct answer. Similar to the previous option, a higher camber value may cause discomfort for drivers and increase tire wear.

Therefore, the correct answer is option 'B' (2.5%), which provides an optimal balance between water drainage and driver comfort on thin bituminous surface hill roads.