The correct answer is C. Lahaul and Spiti.
The percentage of SCs population in Himachal Pradesh is 12.5%. The percentage of SCs population in Lahaul and Spiti is 5.6%, which is the lowest among all the districts of Himachal Pradesh.
The percentage of SCs population in Solan is 13.8%, the percentage of SCs population in Mandi is 13.2%, and the percentage of SCs population in Kullu is 13.3%.
The reason for the low percentage of SCs population in Lahaul and Spiti is that the district has a very small population. The total population of Lahaul and Spiti is only 34,899, which is the lowest among all the districts of Himachal Pradesh. The majority of the population of Lahaul and Spiti is from the Gaddi community, which is a Schedule Tribe.