• The inner membrane of mitochondria forms infoldings called cristae. • Thylakoids are flattened membranous sacs in stroma of plastids. • Cisternae are disc shaped sacs in Golgi apparatus. • Primary constriction in chromosome that holds two chromatids together is called centromere. Hence correct option is (3)- a(iii), b(iv), c(i), d(ii)