The correct answer is: b) Menhirs and dolmens.
Menhirs are single standing stones, while dolmens are megalithic tombs consisting of a large flat stone supported by two or more upright stones. Both menhirs and dolmens are found in Manipur, and are evidence of the prehistoric megalithic culture that existed in the region.
Burial mounds are also found in Manipur, but they are not considered to be evidence of megalithic culture. Burial mounds are simply mounds of earth that were used to bury the dead, and they do not have the same cultural significance as menhirs and dolmens.
Rock shelters are also found in Manipur, but they are not considered to be evidence of megalithic culture either. Rock shelters are simply natural caves or overhangs that were used for shelter, and they do not have the same cultural significance as menhirs and dolmens.
In conclusion, the correct answer to the question “Prehistoric megalithic culture in Manipur is evidenced by the existence of:” is b) Menhirs and dolmens. Menhirs and dolmens are megalithic monuments that are found in Manipur, and they are evidence of the prehistoric megalithic culture that existed in the region.