The correct answer is: a) Secondary burial.
Secondary burial is the practice of burying the dead, then exhuming the body and reburying it at a later date. This practice was common in prehistoric Manipur, and it is thought to have been done for a variety of reasons, including to allow the body to decompose naturally, to remove the body from the place of death, or to allow the deceased to be reunited with their ancestors.
Cremation is the practice of burning the dead. This practice was not common in prehistoric Manipur, but it did occur on occasion. It is thought that cremation was used to prevent the body from decomposing, or to destroy the body so that it could not be used by evil spirits.
Interment in caves is the practice of burying the dead in caves. This practice was not common in prehistoric Manipur, but it did occur on occasion. It is thought that interment in caves was done to protect the body from scavengers, or to provide a place for the deceased to rest in peace.
Exposure to elements is the practice of leaving the dead body exposed to the elements. This practice was not common in prehistoric Manipur, but it did occur on occasion. It is thought that exposure to elements was done to allow the body to decompose naturally, or to prevent the body from being disturbed by animals.
It is important to note that these are just some of the possible reasons why secondary burial was practiced in prehistoric Manipur. There may have been other reasons that are not known or that have not been recorded.