D is the correct answer because the snali have two skeletons one inside the body for ex : human.
and outer skeleton it drags it along its body while moving.

Introduction:
The animal that has its body covered by a hard shell is the snail. The snail is a type of mollusk, characterized by its spiral-shaped shell. This protective covering serves several important functions for the snail.

Body:
1. Snails:
- Snails are gastropod mollusks that belong to the class Gastropoda.
- They are found in a variety of habitats, including terrestrial, freshwater, and marine environments.
- Snails have a soft body, but they are protected by a hard shell that is made up of calcium carbonate.
- The shell is secreted by a specialized organ called the mantle, which is located in the snail's body.

2. Function of the Shell:
- The shell provides protection for the snail's soft body from predators, harsh weather conditions, and physical damage.
- It acts as a barrier against potential threats, such as predators like birds or mammals, by making it difficult for them to access the snail's vulnerable body.
- The shell also helps to prevent desiccation (drying out) in terrestrial snails by providing a moist microenvironment within its walls.
- In aquatic snails, the shell provides buoyancy, allowing them to float or swim in water.

3. Shell Characteristics:
- The shell of a snail is usually coiled and spiraled, which allows the snail to retreat into its shell for protection.
- The shape, size, and color of the shell can vary greatly among different species of snails.
- Some shells have elaborate patterns and colors, serving as camouflage or a warning to potential predators.
- As the snail grows, it adds new layers to its shell, resulting in visible growth rings.

4. Shell Growth and Repair:
- The shell of a snail is a living structure that continues to grow as the snail grows.
- The mantle of the snail secretes new shell material at the edge of the shell, allowing it to gradually increase in size.
- If the shell becomes damaged or worn out, the snail can repair it by secreting additional shell material to reinforce or replace the damaged parts.

Conclusion:
In conclusion, the animal that has its body covered by a hard shell is the snail. The shell serves as a protective covering, providing shelter and defense against predators and environmental factors. The snail's shell is a remarkable adaptation that helps ensure its survival in various habitats.