**Core Concept:** Ossification centers are sites where cartilage is converted into bone during development and growth. In mammals, there are two types of ossification centers: primary and secondary. Primary ossification centers are formed first and are responsible for the initial growth of long bones, while secondary ossification centers, which form later, are responsible for the growth and maintenance of bones.

**Why the Correct Answer is Right:** The correct answer is C, the Ulna. The ulna is a long bone in the forearm, and it has a single primary ossification center at its distal end, which is responsible for its initial growth. In contrast, other options have multiple ossification centers:

**Option A:** Humerus (humerus) has two primary ossification centers for its initial growth and two secondary ossification centers for further growth and maintenance.

**Option B:** Femur (thigh bone) has two primary ossification centers and two secondary ossification centers.

**Option D:** Tibia and Fibula (lower leg bones) have three primary ossification centers and one secondary ossification center.

**Why Each Wrong Option is Incorrect:**

- Option A (Humerus): The humerus has two primary ossification centers, which is incorrect for a single-ossification-center bone.
- Option B (Femur): The femur also has two primary ossification centers, making this option incorrect as well.
- Option D (Tibia and Fibula): These bones have three primary ossification centers and one secondary ossification center, which is inconsistent with the correct number of ossification centers for a single-ossification-center bone.

**Clinical Pearl:** A single ossification center is crucial for understanding the developmental anatomy of bones. It is essential for medical students and doctors to remember the specific bones with single ossification centers in the body, as it helps in diagnosing fractures, pathologies affecting bone growth, and interpreting radiographic images.

**Correct Answer:** C, Ulna. The ulna has a single primary ossification center, which is responsible for its initial growth.