Reptiles were dominant during  

Early in the Triassic, a group of reptiles, the order Ichthyosauria, returned to the ocean. Fossils of early ichthyosaurs are lizard-like and clearly show their tetrapod ancestry. Their vertebrae indicate they probably swam by moving their entire bodies side to side, like modern eels. Later in the Triassic, ichthyosaurs evolved into purely marine forms with dolphin-shaped bodies and long-toothed snouts. Their vertebrae indicate they swam more like fish, using their tails for propulsion with strong fin-shaped forelimbs and vestigial hind limbs. So, reptiles were dominant during triassic period than permian, cambrian and cretaceous period. Thus, option D is correct.