The correct answer is D. Lahaul and Spiti.
Lahaul and Spiti is a district in the Indian state of Himachal Pradesh. It is the largest district in Himachal Pradesh, but it has the lowest population density. The district is located in the Himalayas, and it is a popular tourist destination.
The forest cover in Lahaul and Spiti is about 30%. This is the lowest forest cover of any district in Himachal Pradesh. The main reason for this is that the district is located in a high-altitude region, and the climate is not suitable for trees.
The other districts in Himachal Pradesh have a higher forest cover. Hamirpur has a forest cover of about 60%, Solan has a forest cover of about 50%, and Bilaspur has a forest cover of about 40%.
The forest cover in Himachal Pradesh is important for the environment. The forests help to regulate the climate, and they provide habitat for wildlife. The forests also help to prevent soil erosion.