The correct answer is: B. Sirmaur
Rapeseed and mustard are mostly grown in the Sirmaur district of Himachal Pradesh. The district has a favorable climate for the cultivation of these crops, and the soil is also suitable for their growth. The government of Himachal Pradesh has also been providing subsidies and other incentives to farmers to encourage them to grow rapeseed and mustard. As a result, the production of these crops has increased significantly in the district in recent years.
Kinnaur is a hilly district in the eastern part of Himachal Pradesh. The climate in Kinnaur is cold and dry, and the soil is not suitable for the cultivation of rapeseed and mustard.
Lahaul and Spiti is a high-altitude district in the northern part of Himachal Pradesh. The climate in Lahaul and Spiti is very cold, and the soil is not suitable for the cultivation of rapeseed and mustard.
Chamba is a district in the western part of Himachal Pradesh. The climate in Chamba is moderate, and the soil is suitable for the cultivation of a variety of crops, including rapeseed and mustard. However, the production of rapeseed and mustard in Chamba is not as high as in Sirmaur.