The correct answer is A. Sirmaur.
Sirmaur is a district in Himachal Pradesh, India. It is located in the central part of the state and is bordered by the districts of Solan, Shimla, Kullu, Mandi, and Bilaspur. The district has a population of over 500,000 people and its headquarters is the town of Nahan.
Sirmaur is a major district for floriculture in Himachal Pradesh. The district has a number of cold storage facilities and is well-connected to the major markets in the country. The main flowers grown in Sirmaur are roses, lilies, and carnations.
The other options are incorrect because they are not major districts for floriculture in Himachal Pradesh. Kinnaur is a district in the eastern part of the state and is known for its apple orchards. Lahaul and Spiti are districts in the north-eastern part of the state and are known for their high altitude and cold climate.