The correct answer is A. Chamba.
Chamba district was formed in 1864. It is the oldest district in Himachal Pradesh. It is located in the north-western part of the state. The district has an area of 6,401 square kilometers. The headquarters of the district is Chamba town.
The other options are incorrect because they were formed later than Chamba district. Kinnaur district was formed in 1960, Mandi district was formed in 1966, and Solan district was formed in 1972.
Chamba district is a beautiful and scenic district. It is home to many historical and cultural sites. The district is also a popular tourist destination.