The correct answer is D. Solan.
Solan is a district in the Indian state of Himachal Pradesh. It is located in the central part of the state and is surrounded by the districts of Shimla, Mandi, Sirmaur, and Bilaspur. The district has a population of over 500,000 people and is the headquarters of the Solan division.
Solan is a major agricultural district and is known for its production of apples, potatoes, and mushrooms. The district is also home to a number of industries, including the manufacture of paper, textiles, and pharmaceuticals.
Mushroom growing and canning is a major industry in Solan. The district has a number of mushroom farms and canning factories. The mushrooms grown in Solan are exported to a number of countries, including the United States, Canada, and Europe.
The other options are incorrect because they are not districts in Himachal Pradesh. Kangra is a district in the northern part of the state, Chamba is a district in the western part of the state, and Kinnaur is a district in the eastern part of the state.