Final answer:
b.)Ichthyosaurs are the marine animals that evolved from land reptiles and readapted to aquatic life.
Step-by-step explanation:
The marine animal that evolved from a land reptile and moved back to the water is the Ichthyosaur. Ichthyosaurs were remarkably similar in shape to modern dolphins and sharks, which is a good example of convergent evolution - where unrelated species develop similar traits to adapt to similar environments or niches. The Ichthyosaurs evolved from earlier reptiles that returned to the ocean, similarly to how terrestrial mammals like Rodhocetus led to the evolution of whales.