Final answer:
Amphibians never achieved the evolutionary milestone of developing terrestrially adapted eggs with amniotic membranes like amniotes did, which allowed latter to lay waterproof eggs on land.
Step-by-step explanation:
The major transition that amphibians never made, which was made by amniotes, was the evolution of the terrestrially adapted egg with amniotic membranes. This adaptation allowed amniotes, such as reptiles, birds, and mammals, to reproduce on land by laying eggs that had a waterproof covering to prevent drying out. Unlike amphibians, whose eggs require a moist environment to prevent desiccation, amniotic eggs can retain water, enabling the embryo to breathe and develop in a terrestrial setting, which significantly reduced amniotes' dependence on water for reproduction.