The correct answer is option (D), Hamirpur.
Hamirpur is a district in the Indian state of Himachal Pradesh. It is located in the central part of the state and is bordered by the districts of Kangra to the north, Una to the east, Bilaspur to the south, and Solan to the west. The district has a population of over 1.2 million people and is the third most populous district in Himachal Pradesh.
The rock salt industry is located in the town of Nadaun in Hamirpur district. The industry is a major source of employment for the people of the district. The salt is extracted from the mines in the area and is then processed and packaged for sale. The salt is used for a variety of purposes, including cooking, preserving food, and making medicines.
The other options are incorrect because they are not the locations of the rock salt industry in Himachal Pradesh.
Option (A), Solan, is a town in the Indian state of Himachal Pradesh. It is the headquarters of the Solan district. The town is located in the foothills of the Himalayas and is a popular tourist destination.
Option (B), Kullu, is a district in the Indian state of Himachal Pradesh. It is located in the northern part of the state and is bordered by the districts of Lahaul and Spiti to the north, Mandi to the east, Shimla to the south, and Solan to the west. The district has a population of over 500,000 people and is the second most populous district in Himachal Pradesh.
Option (C), Mandi, is a district in the Indian state of Himachal Pradesh. It is located in the central part of the state and is bordered by the districts of Kangra to the north, Kullu to the east, Shimla to the south, and Hamirpur to the west. The district has a population of over 1 million people and is the fourth most populous district in Himachal Pradesh.