**Core Concept**
Osteomas are benign bone tumors that arise from the cortical bone. They are more commonly found in the paranasal sinuses, particularly in the frontal and ethmoid sinuses.

**Why the Correct Answer is Right**
The frontal sinus is the most common site for osteomas due to its location at the forehead, where the bone is thinner and more prone to tumor development. Osteomas in the frontal sinus often present with symptoms such as headaches, facial pain, and visual disturbances. The frontal sinus's proximity to the orbit and the brain makes it a critical area for osteoma growth.

**Why Each Wrong Option is Incorrect**
**Option A:** Maxillary sinus osteomas are less common than those found in the frontal or ethmoid sinuses.
**Option B:** Sphenoid sinus osteomas are extremely rare and typically present with more severe symptoms due to their location near critical neurovascular structures.
**Option C:** Ethmoid sinus osteomas are more common than those found in the maxillary sinus but less common than those in the frontal sinus.

**Clinical Pearl / High-Yield Fact**
Osteomas can be asymptomatic for years, and their diagnosis often relies on imaging studies such as CT scans. A high index of suspicion is necessary for diagnosing osteomas, especially in patients with recurrent sinusitis or facial pain.

**Correct Answer: C. Ethmoid sinus.**