**Core Concept**
Osteoma is a benign bone tumor that can occur in any bone, including the paranasal sinuses. The paranasal sinuses are air-filled cavities located within the bones of the face, and osteomas can cause a range of symptoms depending on their location and size.

**Why the Correct Answer is Right**
The frontal sinus is the most common paranasal sinus affected by osteoma. This is likely due to the fact that the frontal sinus is the largest of the paranasal sinuses and is located in a region where bone is relatively thin and susceptible to tumor formation. Osteomas in the frontal sinus can cause symptoms such as sinusitis, headaches, and facial pain, and may require surgical removal.

**Why Each Wrong Option is Incorrect**
* **Option A:** Ethmoid sinus - While osteomas can occur in the ethmoid sinus, it is not the most common location. The ethmoid sinus is a complex structure with many small air cells, making it less likely to be affected by a single osteoma.
* **Option B:** Maxillary sinus - The maxillary sinus is the second largest paranasal sinus, but it is less commonly affected by osteoma than the frontal sinus. The maxillary sinus is also more prone to other types of tumors, such as squamous cell carcinoma.
* **Option C:** Sphenoid sinus - The sphenoid sinus is the smallest of the paranasal sinuses and is less commonly affected by osteoma. The sphenoid sinus is also a more complex structure with many surrounding vital structures, making it more difficult to treat.

**Clinical Pearl / High-Yield Fact**
Osteomas in the paranasal sinuses are often asymptomatic and may not be discovered until they cause symptoms or are found incidentally on imaging studies. A high index of suspicion is necessary to diagnose osteoma, particularly in patients with recurrent sinusitis or facial pain.

**Correct Answer:** A. Frontal sinus.