## Core Concept
The pneumatization of the maxillary sinus is a process where the sinus cavity expands into the surrounding bone, influenced by the growth and development of the facial bones and sinuses during childhood and adolescence. This process involves the resorption of bone and the formation of air cells within the maxillary bone.

## Why the Correct Answer is Right
The pneumatization of the maxillary sinus is known to be completed around the age of 6 years. This process starts early in life and continues to develop as the child grows. By the age of 6, the maxillary sinus has reached a significant size and its pneumatization is considered complete in terms of its basic structural development, although it continues to grow in size.

## Why Each Wrong Option is Incorrect
* **Option A:** This option suggests an earlier age, which does not align with the completion of pneumatization.
* **Option B:** This option suggests a later age, which might be too late for the completion of the basic pneumatization process.
* **Option D:** This option also suggests a different age, which does not match the established timeline for the completion of pneumatization.

## Clinical Pearl / High-Yield Fact
A key point to remember is that the maxillary sinuses are the first paranasal sinuses to pneumatize, and their development can be an indicator of overall facial growth and sinus health. Clinicians often consider the pneumatization of sinuses when evaluating sinusitis or other sinus-related conditions in pediatric patients.

**Correct Answer: C. 6 years.**