The correct answer is: d) All of the above
Caves and rock shelters are often used as habitation sites by humans due to their natural protection from the elements. River valleys are also important archaeological sites, as they provide a source of water and food, as well as a means of transportation. High-altitude passes are also potential archaeological sites, as they were often used as trade routes between different cultures.
Caves and rock shelters can provide valuable information about the past, as they can contain evidence of human habitation, such as tools, artifacts, and remains. River valleys can also provide valuable information about the past, as they can contain evidence of human activity, such as settlements, agricultural terraces, and irrigation systems. High-altitude passes can also provide valuable information about the past, as they can contain evidence of human activity, such as trade routes, military fortifications, and religious shrines.
In Sikkim, there are many potential archaeological sites that have yet to be explored. These sites could provide valuable information about the history and culture of Sikkim.