The correct answer is: C. Sirmaur
Gypsum is mostly found in the Sirmaur district of Himachal Pradesh. The district is home to several large gypsum mines, including the Deoghar Gypsum Mine and the Renuka Gypsum Mine. Gypsum is a mineral that is used in a variety of applications, including construction, agriculture, and manufacturing.
The other options are incorrect. Mandi is a district in Himachal Pradesh that is known for its apple orchards. Kullu is a district in Himachal Pradesh that is known for its tourism industry. Solan is a district in Himachal Pradesh that is known for its educational institutions.