Ichthyosaurs were:

The correct answer is Option (1) → Fish-like reptiles that evolved back into water around 200 mya.

Reptiles of many shapes and sizes ruled the earth for about 200 million years. Most of them lived on land, but some land reptiles went back into water. There they evolved a fish-like body, probably about 200 million years ago (mya). Ichthyosaurs are the book’s example of these fish-like reptiles.

Option (1) Fish-like reptiles that evolved back into water around 200 mya. — Correct. Ichthyosaurs were reptiles that came from land reptiles and returned to water, about 200 mya.

Option (2) Land reptiles that evolved into birds. — Incorrect. The land reptiles were the dinosaurs. The book says some people think most dinosaurs evolved into birds. Ichthyosaurs lived in water and are not linked to birds.

Option (3) Mammals that dominated the land. — Incorrect. Mammals took over the land only after the reptiles declined. Ichthyosaurs were reptiles, not mammals.

Option (4) Ancestors of modern amphibians. — Incorrect. The ancestors of amphibians were the lobefin fish, such as the coelacanth group. Ichthyosaurs came much later, from reptiles.

NCERT Extract:
A Brief Account of Evolution – “… In the next 200 millions years or so, reptiles of different shapes and sizes dominated on earth. Giant ferns (pteridophytes) were present but they all fell to form coal deposits slowly. Some of these land reptiles went back into water to evolve into fish like reptiles probably 200 mya (e.g. Ichthyosaurs). The land reptiles were, of course, the dinosaurs.”