**Core Concept**
The Haversian system is a microscopic structure found in compact or cortical bone, which is the dense outer layer of bone tissue. It is characterized by a central Haversian canal surrounded by concentric layers of bone tissue called lamellae.

**Why the Correct Answer is Right**
The Haversian system is a key feature of cortical bone, allowing for the passage of nerves, blood vessels, and lymphatic vessels through the bone. Within the Haversian canal, the Haversian vessels, also known as the central artery and veins, supply the surrounding bone tissue with oxygen and nutrients. The lamellae, composed of compact bone tissue, provide structural support and protection to the bone. The Haversian system is essential for the maintenance of bone health and the regulation of bone metabolism.

**Why Each Wrong Option is Incorrect**
**Option B:** Cancellous bone, also known as spongy bone, lacks the organized structure of the Haversian system and instead consists of a network of trabeculae.
**Option C:** Teeth have a unique structure, with enamel, dentin, and pulp, but they do not possess a Haversian system.
**Option D:** Nails are made of keratin and do not have a bony structure, thus they do not contain Haversian systems.

**Clinical Pearl / High-Yield Fact**
The Haversian system is an important consideration in orthopedic surgery, as damage to these structures can lead to bone necrosis and other complications.

**✓ Correct Answer: A. Cortical bone**