A distinct feature of birds is A. Feathers
B. Claws
C. Beaks without teeth
D. Wings

Hint: Birds are called avians where they have special adaptation compared to other classes because they have ability to flight, first formed birds have characters related to reptiles but after variations the modern birds lost features that first formed birds had.

Complete answer:
Salient features of Aves are:
-The birds do not have mouths and teeth and the jaws of them are covered with the beak helping them to pick the food.
-They have special feathers or wings for the purpose of flying and these are warm blooded animals.
-Birds have air cavities inside the body which helps them to maintain buoyancy in air to fly.
-Body parts include head, neck and tail regions.
-The forelimbs are modified as wings which helps in flying and the limbs are paired.
-The sweat glands are absent and the oil glands are located in tails.
-Birds are egg laying species with four chambered hearts in them.
-The ear pinna are undeveloped in nature.
-The examples of the birds belonging to the class Aves are sparrow, crow, pigeons, ducks, doves, flamingo etc. Ostrich is the only bird which cannot fly and it belongs to class Aves. Birds are modified into features whereas claws, beaks without teeth and wings are seen in the reptiles and mammals.

Hence, the correct answer is option (A).

Note: The aves are classified into two types: Archaeornithes and Neornithes. Archaeornithes are extinct birds with long tails and teeth whereas Neornithes are without teeth and long tails like penguins.