The Border Roads Organisation (BRO) was established with the primary objective of constructing and maintaining roads in high altitude mountainous terrains, particularly along India's borders. This includes regions in the Himalayan and northeastern areas where road construction is particularly challenging due to the difficult terrain and strategic importance.
Option (B) correctly identifies the main role of the BRO, while the other options focus on regions that are not its primary focus.
Thus, the correct answer is (B) To construct roads in high altitude mountainous terrains.