Answer:
Amniotic egg.
Explanation:
Amniotic egg is the advanced evolutionary character first appeared in reptiles. These amniotic egg allows the reptiles to colonize on the dry lands. This amniotic egg is covered by hard shell in birds and reptiles.
The amniotic egg is not present in fishes and frogs as they lay their eggs in water. The amniotic egg is present in reptiles, birds and mammals. The vertebral column, paired appendages, jaws and digits are present in frogs, reptiles, birds and mammals.
Thus, the correct answer is option (a).
