The correct answer is: a) Ancestor worship.
Megaliths are large stones that have been erected by humans. They are found all over the world, and they can be used for a variety of purposes. In Manipur, megaliths are often associated with ancestor worship. They are believed to be the homes of the dead, and they are often used as places of worship.
Megaliths can also be used for agricultural rituals. They can be used to mark the boundaries of fields, or they can be used to mark the beginning of the planting season. In some cases, megaliths are also used as defensive structures. They can be used to protect villages from attack, or they can be used to mark the boundaries of territory.
However, in Manipur, megaliths are most commonly associated with ancestor worship. They are believed to be the homes of the dead, and they are often used as places of worship.