**Core Concept**
The human skull at birth is mostly composed of cartilaginous structures, which gradually ossify and fuse together as the infant grows. This process is crucial for the development of the craniofacial complex and the formation of the adult skull.

**Why the Correct Answer is Right**
The correct answer is related to the fact that the human skull at birth is composed of 45% cartilage and 55% bone. The cartilaginous structures, such as the fontanelles and the nasal septum, begin to ossify and fuse together, eventually replacing the cartilage with bone. This process is facilitated by the presence of osteoblasts, which are responsible for bone matrix deposition, and osteoclasts, which resorb cartilage and bone.

**Why Each Wrong Option is Incorrect**
**Option A:** This option is incorrect because it does not accurately describe the composition of the human skull at birth.
**Option B:** This option is incorrect because it is too vague and does not provide any specific information about the skull's composition.
**Option C:** This option is incorrect because it is not a well-known fact about the human skull at birth.

**Clinical Pearl / High-Yield Fact**
It's essential to remember that the human skull at birth contains fontanelles, which are areas of cartilage that eventually ossify and fuse together as the infant grows. Failure of these fontanelles to close can lead to various skull deformities and neurological issues.

**Correct Answer: D. 55% bone**