Exoskeleton of hair, external ears and mostly giving birth to live young is characteristic features of

The correct option is D

Mammalia

The correct option is D.

Explanation of the correct option :

1. Mammals are warm-blooded vertebrates.
2. Their entire body is covered with hair, which helps them to adapt to their environment.
3. Their skin possesses oil and sweat glands as well.
4. External ears or Pinnae are present in most mammals and are also aided with 3 middle ear bones.
5. They are characterized mostly by the presence of mammary glands and are viviparous, giving birth to young ones.

Explanation of the incorrect option :

Option A:

Aves are warm-blooded birds with their forelimbs modified into wings for flight.

Option B:

Reptiles have their skin covered with thick scales and are oviparous.

Option C:

Pisces are the fishes, whose body is covered with smooth, slimy scales and streamlined body with fins.

Final answer: Exoskeleton of hair, external ears and mostly giving birth to live young are characteristic features of mammals.