**Core Concept**
Sinus development is a complex process that begins in utero and continues into early childhood. The paranasal sinuses are air-filled cavities that develop from outgrowths of the nasal cavity and are essential for resonance and support of facial structures.

**Why the Correct Answer is Right**
The ethmoid sinuses are the first to form, appearing as early as 8-10 weeks of gestation. They develop from the lateral nasal prominences and the nasal septal crest. The frontal, maxillary, and sphenoid sinuses develop later, around the 20th week of gestation. The maxillary sinuses are the largest of the paranasal sinuses and are present at birth, while the frontal and sphenoid sinuses continue to develop postnatally.

**Why Each Wrong Option is Incorrect**
* **Option A:** Not specified, but some options may be incorrect or incomplete.
* **Option B:** Not correct, as the ethmoid sinuses are present at birth but not the only sinuses.
* **Option C:** Not correct, as the maxillary sinuses are indeed present at birth, but this option may be incomplete or incorrect.

**Clinical Pearl / High-Yield Fact**
The paranasal sinuses are fully developed by around 6-7 years of age, which is important for pediatricians and primary care physicians to remember when evaluating children with respiratory issues.

**Correct Answer: C. Maxillary sinuses.**