**Core Concept**
The formation of a radio-opaque secondary in bone is a process involving the deposition of minerals, particularly calcium and phosphate, onto a matrix of collagen fibers. This process is characteristic of bone growth and remodeling, where osteoblasts lay down new bone matrix and mineralize it.

**Why the Correct Answer is Right**
The correct answer is **A. Osteoblasts**. Osteoblasts are responsible for the production of new bone matrix, which is composed of collagen fibers and minerals. As the bone matrix is produced, minerals such as calcium and phosphate are deposited onto the matrix, resulting in the formation of a radio-opaque secondary in bone. This process is essential for bone growth, remodeling, and repair.

**Why Each Wrong Option is Incorrect**
**Option B:** Osteoclasts are responsible for bone resorption, not bone production, so they would not produce a radio-opaque secondary in bone.

**Option C:** Chondrocytes are involved in cartilage production, not bone production, so they would not produce a radio-opaque secondary in bone.

**Option D:** There is no option D provided in the question.

**Clinical Pearl / High-Yield Fact**
It's essential to remember that the formation of a radio-opaque secondary in bone is a dynamic process involving the coordinated action of osteoblasts and osteoclasts. Osteoblasts produce new bone matrix, which is then mineralized by the deposition of calcium and phosphate, while osteoclasts resorb existing bone tissue to facilitate bone remodeling.

**Correct Answer:** A. Osteoblasts.