The correct answer is: B. Lahaul and Spiti
Lahaul and Spiti is a district in the Indian state of Himachal Pradesh. It is the least populous district in the state, with a population of just over 30,000 people. The district is located in the Himalayas, and its terrain is largely mountainous. The majority of the population of Lahaul and Spiti lives in rural areas, with only a small percentage living in urban areas.
The other options are incorrect because they are all districts in Himachal Pradesh with a higher rural population than Lahaul and Spiti. Kullu has a population of over 500,000 people, of which over 400,000 live in rural areas. Hamirpur has a population of over 1 million people, of which over 800,000 live in rural areas. Una has a population of over 500,000 people, of which over 400,000 live in rural areas.