The correct answer is D. Kinnaur.
Kinnaur is a district in the Indian state of Himachal Pradesh. It is the least populous district in the state, with a population of 88,990 as of 2011. The district has a total area of 6,401 square kilometers.
The road length in Kinnaur is 1,096 kilometers. This is the least among all the districts in Himachal Pradesh. The reason for this is that Kinnaur is a mountainous district with difficult terrain. It is also a remote district, with limited access to roads and other infrastructure.
The other options are incorrect because they have a higher road length than Kinnaur. Kangra has a road length of 4,279 kilometers, Kullu has a road length of 2,922 kilometers, and Una has a road length of 2,509 kilometers.
I hope this helps!