The diagram shows a vertebrate. To which vertebrate group does it belong?

The diagram shows an animal with fur, four legs, a tail, and whiskers, resembling a mouse or rat.

• Amphibians (e.g., frogs, salamanders) typically have moist skin, lay eggs in water, and undergo metamorphosis.
• Birds (e.g., sparrows, eagles) have feathers, wings, beaks, and lay hard-shelled eggs.
• Fish (e.g., sharks, salmon) live in water, have gills, fins, and scales.
• Mammals (e.g., humans, dogs, mice) have fur or hair, are warm-blooded, give birth to live young, and feed their young milk.

The animal in the diagram has fur and is clearly a terrestrial animal that gives birth to live young and feeds them milk, which are defining characteristics of mammals.

The animal belongs to the group Mammal.

The correct option is D.
$\boxed{\text{D Mammal}}$