The correct answer is: A. Sirmaur
Sirmaur district has the highest percentage of SCs population in Himachal Pradesh. According to the 2011 census, the SC population in Sirmaur district is 26.6%, which is higher than the state average of 21.9%.
The other options are incorrect because they have a lower percentage of SCs population than Sirmaur district. Kullu district has an SC population of 19.4%, Bilaspur district has an SC population of 20.9%, and Shimla district has an SC population of 21.3%.
The SC population in Himachal Pradesh is concentrated in the southern and eastern parts of the state. The districts with the highest percentage of SCs population are Sirmaur, Solan, Kangra, and Hamirpur. The districts with the lowest percentage of SCs population are Lahaul and Spiti, Kinnaur, and Chamba.
The SC population in Himachal Pradesh is mainly engaged in agriculture and allied activities. A significant proportion of the SC population is also engaged in government service and other non-agricultural occupations.