The correct answer is: a) Stupa
A stupa is a mound-like structure that is used as a Buddhist monument. It is typically built over the remains of a Buddhist saint or over an object that is considered to be sacred. Stupas are often decorated with images of the Buddha and other Buddhist symbols.
A dolmen is a type of megalithic structure that consists of two or more upright stones supporting a horizontal stone. Dolmens are often found in groups, and they are thought to have been used as tombs or as places of worship.
A menhir is a single upright stone that is often found in megalithic monuments. Menhirs are thought to have been used for a variety of purposes, including as markers of sacred sites, as boundary markers, and as astronomical observatories.
A ziggurat is a type of stepped pyramid that was built by the ancient Mesopotamians. Ziggurats were used as temples, and they were often decorated with brightly colored tiles.
In conclusion, a stupa is a megalithic structure associated with ancestor worship. It is a mound-like structure that is used as a Buddhist monument.