The correct answer is: All of these.
Solan, Una, and Hamirpur were all founded in 1972.
Solan 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 Shimla, Sirmaur, Mandi, Kullu, and Bilaspur. The district has a population of over 500,000 people, and its headquarters is the town of Solan.
Una is a district in the Indian state of Himachal Pradesh. It is located in the southern part of the state, and is bordered by the districts of Bilaspur, Hamirpur, Kangra, and Chamba. The district has a population of over 600,000 people, and its headquarters is the town of Una.
Hamirpur is a district in the Indian state of Himachal Pradesh. It is located in the western part of the state, and is bordered by the districts of Kangra, Una, Bilaspur, and Solan. The district has a population of over 700,000 people, and its headquarters is the town of Hamirpur.
All three districts were formed in 1972, when the state of Himachal Pradesh was created.