**Core Concept**
The maxillary sinus is a paranasal sinus that plays a crucial role in the drainage of the nasal cavity. It is one of the four paired paranasal sinuses, and its anatomy is essential for understanding its function and clinical significance.

**Why the Correct Answer is Right**
The maxillary sinus opens into the middle meatus of the nasal cavity through the semilunar hiatus. This opening is a critical structure for the drainage of the sinus and allows for the exchange of air and mucus between the sinus and the nasal cavity. The semilunar hiatus is a small, crescent-shaped opening that is formed by the uncinate process and the ethmoid bulla.

**Why Each Wrong Option is Incorrect**
**Option A:** The frontal sinus does not directly communicate with the maxillary sinus.
**Option B:** The sphenoid sinus opens into the sphenoethmoidal recess, not the maxillary sinus.
**Option C:** The ethmoid air cells do not have a direct opening into the maxillary sinus; they drain into the middle meatus through separate openings.

**Clinical Pearl / High-Yield Fact**
The maxillary sinus is the largest of the paranasal sinuses and is responsible for producing a significant portion of the nasal mucus. Understanding its anatomy and function is essential for the diagnosis and treatment of sinusitis and other maxillary sinus disorders.

**Correct Answer:** C.