**Core Concept**
Fibrous joints (synarthroses) are immovable joints held together by dense regular connective tissue, specifically fibrous connective tissue with collagen fibers. These joints are characterized by the absence of articular cartilage and synovial fluid, and are classified as sutures or gomphoses in the skeletal system.

**Why the Correct Answer is Right**
The fronto-parietal suture is a fibrous joint located between the frontal and parietal bones of the skull. It is a true suture, formed by dense fibrous connective tissue, allowing minimal movement and providing structural rigidity to the skull. This joint is a classic example of a synarthrotic fibrous joint, where bones are joined by fibrous tissue without cartilage or synovial fluid.

**Why Each Wrong Option is Incorrect**
Option A: Pubic symphysis is a **symphysis**, a type of cartilaginous joint, not fibrous. It contains fibrocartilage, not dense fibrous tissue.
Option C: Manubrio-sternal joint is a **synchondrosis** (cartilaginous joint), where the manubrium and sternum are joined by hyaline cartilage.
Option D: Infra-radio-ulnar joint is a **synovial joint** (hinge joint), allowing free movement, and is not fibrous.

**Clinical Pearl / High-Yield Fact**
Remember: **Sutures = fibrous joints**, and they are only found in the skull. The most common fibrous joint in the body is the **fronto-parietal suture**. Always check whether the joint has fibrous tissue, cartilage, or synovial fluid to classify it.

✓ Correct Answer: B. Fronto parietal suture