## **Core Concept**
The basioccipital and basisphenoid bones are part of the skull's base. They are embryological precursors that fuse during development to form the **clivus**, a critical structure in the posterior cranial fossa.

## **Why the Correct Answer is Right**
The basioccipital and basisphenoid bones fuse to form the **sphenoccipital synchondrosis**, which is a cartilaginous joint. This fusion occurs at around **25 years of age** in humans. The sphenoccipital synchondrosis is crucial for the growth of the skull base. Once fused, it forms a solid bony structure that contributes to the clivus.

## **Why Each Wrong Option is Incorrect**
- **Option A:** This option is incorrect because the fusion does not occur in early childhood.
- **Option B:** This option is incorrect because the fusion is not completed during late adolescence; it happens later.
- **Option C:** This option is incorrect because it does not accurately represent the age of fusion.

## **Clinical Pearl / High-Yield Fact**
A key point to remember is that the **sphenoccipital synchondrosis** closes at around **25 years of age**. This is significant in the context of skull growth and trauma, as the vulnerability of the skull base changes with age.

## **Correct Answer:** D. 25 years.