**Core Concept**
The fontanelles are soft spots on a baby's skull where the bones have not yet fused together. They are essential for the baby's growth and development, allowing for the expansion of the brain and the movement of the bones as the baby grows.

**Why the Correct Answer is Right**
The posterior fontanelle, also known as the posterior fontanelle of the occipital bone, is the last to close due to its location and function. It is situated at the back of the skull and is responsible for the expansion and growth of the occipital bone. The posterior fontanelle typically closes between 1-3 months of age, whereas the other fontanelles close earlier in infancy.

**Why Each Wrong Option is Incorrect**
**Option A:** The anterior fontanelle, also known as the soft spot on the top of the head, is the first to close, typically by 18 months of age. It is not the last to close.
**Option B:** The sphenoidal fontanelle, located between the sphenoid and temporal bones, is not a typical fontanelle and is not the last to close.
**Option C:** The mastoid fontanelle, located behind the ear, is not a typical fontanelle and is not the last to close.

**Clinical Pearl / High-Yield Fact**
It is essential for pediatricians and healthcare providers to monitor the fontanelles of infants to detect any potential issues, such as premature closure or delayed closure, which can be indicative of underlying conditions.

**Correct Answer:** C.