Shark skeletons are very different from those of bony fish and terrestrial vertebrates. Sharks and other cartilaginous fish (skates and rays) have skeletons made of cartilage and connective tissue. Cartilage is flexible and durable, yet is about half the normal density of bone.

The correct answer is option 'C', shark.

Sharks are a type of fish that have a unique skeletal structure made entirely of cartilage. Cartilage is a tough and flexible connective tissue that is found in various parts of the body, including the ears, nose, and joints. In sharks, cartilage replaces the need for a bony skeleton, making them different from most other fish and animals.

Reasons why sharks have a cartilaginous skeleton:

1. Adaptation to the aquatic environment:
- The cartilaginous skeleton is lighter than a bony skeleton, allowing sharks to be more buoyant in the water.
- This buoyancy helps sharks swim more efficiently and with less energy expenditure.

2. Flexibility and maneuverability:
- Cartilage is more flexible than bone, allowing sharks to bend and twist their bodies in order to navigate through the water more easily.
- This flexibility also enables sharks to make quick and agile movements, which is crucial for their hunting and survival.

3. Protection and shock absorption:
- Although cartilage is not as strong as bone, it still provides some protection to the vital organs of sharks.
- Additionally, cartilage is more resistant to damage from impact and helps absorb shocks that may occur during hunting or encounters with other animals.

4. Growth and development:
- A cartilaginous skeleton allows for faster growth and development in sharks compared to animals with a bony skeleton.
- Cartilage is more easily formed and remodeled, allowing young sharks to grow quickly and adapt to their changing environment.

In summary, sharks have a cartilaginous skeleton because it provides advantages in terms of buoyancy, flexibility, protection, and growth. This unique adaptation has allowed sharks to be successful predators in the aquatic environment for millions of years.

Shark is the aquatic animal whose skeleton is made up of cartilages.