Correct option is (a) constant centre arch dam
Best explanation: In a constant radius arch dam, the centres of extrados, intrados as well as the centrelines of the horizontal arch rings lie on a straight vertical line at various elevations that passes through the centre of the horizontal arch ring at the crest. The centre of the arch rings is not at one point but lies along on vertical line at different heights where in variable centre arch dam the centres do not lie on the same vertical line.