Animals faced challenges when moving from water to land. What was the single most important adaptation that led to the uncoupling of reproduction and the need for an aquatic habitat?

Amniotic egg

Explanation:

The first fully terrestrial vertebrates (adapted to land) were amniotes meaning that they produce eggs with internal membranes that enable embryo breathing without drying out (gases could pass through, but not the water).

For example, amphibians were the first animals on the land, but they had to return to water to reproduce, because they lack amniotic egg.