**Core Concept**
Posterior fontanelles are a type of soft spot on the infant's skull, typically located at the back of the head. The process of ossification, or bone formation, is a critical aspect of skull development in infants.

**Why the Correct Answer is Right**
The posterior fontanelle is the last fontanelle to close, usually by 6 months of age. This is because the parietal bones, which form the sides and back of the skull, have a complex and irregular shape, making the process of bone closure more time-consuming. The premature closure of the posterior fontanelle can be a sign of increased intracranial pressure or other serious conditions.

**Why Each Wrong Option is Incorrect**
**Option A:** This option is incorrect because the anterior fontanelle, not the posterior fontanelle, is the first to close, typically by 18 months of age.
**Option B:** This option is incorrect because the posterior fontanelle does not close until at least 6 months of age, and often remains open until a child is 1-2 years old.
**Option C:** This option is incorrect because the closure of the posterior fontanelle is a gradual process, and there is no specific age at which it occurs.

**Clinical Pearl / High-Yield Fact**
It's essential for healthcare providers to monitor the closure of fontanelles in infants, as delayed or premature closure can be indicative of underlying conditions such as hydrocephalus, craniosynostosis, or increased intracranial pressure.

**Correct Answer: D. 6 months.**