The correct answer is: d) All of the above.
Limited archaeological exploration in many parts of Assam means that our understanding of prehistory is incomplete, many sites might remain undiscovered, and there are gaps in our knowledge.
Archaeology is the study of human history through the excavation of material remains. It is a valuable tool for understanding the past, but it is limited by the fact that it can only study sites that have been excavated. In many parts of Assam, there has been limited archaeological exploration, which means that many sites have not been excavated and their contents are unknown. This limits our understanding of prehistory in Assam.
Even when sites are excavated, not all of the material remains are recovered. Some materials may be destroyed by natural processes, such as erosion or flooding. Others may be lost or damaged during the excavation process. This means that there are always gaps in our knowledge of the past, even when sites have been excavated.
The limited archaeological exploration in many parts of Assam, combined with the fact that not all of the material remains from excavated sites are recovered, means that our understanding of prehistory in Assam is incomplete. There are many sites that have not been excavated, and even the sites that have been excavated do not provide a complete picture of the past. This is why archaeological research is ongoing, and why new discoveries are being made all the time.