The correct answer is: A. Una
The Swan river is located in the Una district of Himachal Pradesh. It is a tributary of the Beas river. The river originates in the Dhauladhar mountains and flows through the Una district before joining the Beas river near the town of Una. The Swan river is a popular tourist destination and is known for its scenic beauty.
Option B, Kinnaur, is incorrect because the Swan river is not located in the Kinnaur district. The Kinnaur district is located in the eastern part of Himachal Pradesh and is bordered by the states of Uttarakhand and China. The main rivers in the Kinnaur district are the Sutlej river and the Spiti river.
Option C, Sirmaur, is incorrect because the Swan river is not located in the Sirmaur district. The Sirmaur district is located in the southern part of Himachal Pradesh and is bordered by the states of Haryana and Uttarakhand. The main rivers in the Sirmaur district are the Yamuna river and the Giri river.
Option D, Solan, is incorrect because the Swan river is not located in the Solan district. The Solan district is located in the central part of Himachal Pradesh and is bordered by the states of Haryana and Punjab. The main rivers in the Solan district are the Beas river and the Sutlej river.