Which of the following is a 3rd century BC structure ?

Mahasthangarh (Bengali: মহাস্থানগড় Môhasthangôṛ) is one of the earliest urban archaeological sites so far discovered in Bangladesh. The village Mahasthan in Shibganj thana of Bogra District contains the remains of an ancient city which was called Pundranagara or Paundravardhanapura in the territory of Pundravardhana. A limestone slab bearing six lines in Prakrit in Brahmi script recording a land grant, discovered in 1931, dates Mahasthangarh to at least the 3rd century BC. The fortified area was in use until the 8th century AD.