The correct answer is: a) Border areas with Himachal Pradesh.
The reason for this is that the border areas with Himachal Pradesh are home to a number of tribal groups, such as the Gaddis, the Kinnauris, and the Lahaulis. These groups have a long history of living in these areas, and they have a strong cultural identity.
The Shivalik foothills are also home to a number of tribal groups, but they are not as concentrated in this area as they are in the border areas with Himachal Pradesh. The industrial zones of Punjab are home to a large number of people from all over the state, and there is no particular concentration of ST population in these areas. The central plains of Punjab are also home to a large number of people from all over the state, and there is no particular concentration of ST population in these areas.
In conclusion, the border areas with Himachal Pradesh are the region of Punjab that might have a relatively higher concentration of ST population.