A distinguishing feature of mammals is the possession of

Step 1: Analyze the question We need the feature that sets mammals apart from other vertebrate groups (fish, amphibians, reptiles, birds).

Step 2: Explain the concept Mammals (class Mammalia) have these unique characteristics:

• Hair or fur on the body at some stage of life
• Mammary glands that produce milk to feed the young
• A diaphragm for breathing
• External ears (pinnae) in most species
• Three middle-ear bones

Step 3: Evaluate each option

• A. Skin: All vertebrates have skin, so it is not distinctive to mammals.
• B. Scale: Scales are found on fish and reptiles (and on bird legs). Mammals do not generally have them (pangolins are a rare exception, and their scales are made of keratin).
• C. Nail: Nails, claws and hooves also occur in reptiles and birds (claws), so nails are not exclusive to mammals.
• D. Hair: Only mammals have true hair. Even whales and dolphins have some hair at some stage of life.

Step 4: Answer The correct option is D. Hair.

Tip: Remember "Mammals = Milk + Mane (hair)". If a question asks for a distinguishing feature of mammals, think of hair and mammary glands first. For birds, think feathers; for reptiles, dry scaly skin; for fish, gills and scales.