In the diagram, A unique characteristic that distinguishes the class of vertebra...

Chickens belong to the class Aves, which includes all birds. A unique characteristic that distinguishes birds (class Aves) from other vertebrates is the presence of a covering of feathers. Feathers provide insulation, aid in flight, and contribute to visual signaling and camouflage.

While some of the other options might seem like unique characteristics, they are not exclusive to birds. For example, two pairs of limbs are present in many other vertebrates, like mammals and reptiles. Scales on the lower legs are also found in reptiles, and a pair of wings, though common in birds, can also be found in bats, which are mammals.

Therefore, the correct answer is Option D: a covering of feathers, as this is the distinguishing feature of birds in the class Aves compared to other vertebrates.