**Core Concept**
The nasal meatuses are air-filled passages in the nasal cavity, and each has specific structures that open into them. The middle meatus is one of these passages, and it contains important openings related to the **paranasal sinuses**.

**Why the Correct Answer is Right**
The correct answer is related to the anatomy of the nasal cavity, specifically which structures open into the middle meatus. The **anterior ethmoidal cells**, **frontal sinus**, and **maxillary sinus** all have openings that communicate with the middle meatus through the **semilunar hiatus**.

**Why Each Wrong Option is Incorrect**
**Option A:** Incorrect because the frontal sinus does indeed open into the middle meatus.
**Option B:** Incorrect as the anterior ethmoidal cells also open here.
**Option C:** Incorrect because the maxillary sinus opens into the middle meatus.

**Clinical Pearl / High-Yield Fact**
It's crucial to remember that the middle meatus is a key area for drainage of the **paranasal sinuses**, and obstruction here can lead to sinusitis.

**Correct Answer:** D. Sphenoid sinus.