Q.Birds and mammals share one of the following characteristics as a common feature.

The correct option is (D) Warm blooded.

• Birds are explicitly warm-blooded (homoiothermous), able to keep body temperature constant.
• Mammals are also explicitly homoiothermous.
• Pneumatic (air-filled) bones are a bird-specific flight adaptation, not shared by mammals. …

Birds and mammals are the two most advanced vertebrate classes, and they are grouped together as the only classes able to keep their body temperature constant regardless of the surroundings.

• Pneumatic bones (B) — hollow, air-filled bones that reduce weight — are a flight-related adaptation unique to birds; mammals do not have this feature.
• Viviparity (C) — giving birth to live young — is typical of most mammals, but birds are oviparous, laying eggs, so this is not shared.
• Pigmented skin (A) is not stated as a defining shared character for either class in their description. …

Method: Venn-diagram-of-features approach

Draw a separate feature list for birds and for mammals, then take the overlap (intersection) of the two lists, rather than checking each option against memory one at a time.

Bird feature list: feathers, wings from forelimbs, pneumatic (hollow) bones, four-chambered heart, warm-blooded (homoiothermous), oviparous.

Mammal feature list: hair, mammary glands, four-chambered heart, warm-blooded (homoiothermous), mostly viviparous.

Take the intersection — features appearing on BOTH lists:

• Pneumatic bones: appears only on the bird list (a flight-specific weight adaptation) — mammals do not have hollow, air-filled bones. Not in the intersection.
• Viviparity: appears only on the mammal list — birds are oviparous, not viviparous. Not in the intersection.
• Four-chambered heart: appears on both lists, so it IS a genuine overlap — but the question offers "warm blooded" as the specific option to check, not heart chambers.
• Warm blooded (homoiothermous): appears on BOTH lists explicitly — both classes actively regulate and maintain a constant internal body temperature regardless of surroundings. …