The correct answer is: The densely forested areas of South Sikkim.
The densely forested areas of South Sikkim are a potential treasure trove of archaeological discoveries related to prehistory. The region is home to a number of ancient sites, including caves, rock shelters, and megalithic monuments. These sites have the potential to provide valuable insights into the lives of the people who lived in Sikkim in prehistoric times.
The high-altitude Tsomgo Lake region is also a potential area for archaeological discoveries. However, the region is difficult to access and the climate is harsh, which makes it less likely that significant archaeological discoveries will be made there.
The banks of the Teesta River are another potential area for archaeological discoveries. However, the region is heavily populated and the river is constantly changing course, which makes it difficult to find and study archaeological sites.
The border areas with Nepal are also a potential area for archaeological discoveries. However, the region is remote and the terrain is difficult to traverse, which makes it difficult to conduct archaeological research there.
In conclusion, the densely forested areas of South Sikkim are the most likely region of Sikkim to hold significant potential for future archaeological discoveries related to prehistory.