**Core Concept**
Haller cells are mucous-containing, air-filled cavities located in the infratemporal fossa, associated with the maxillary sinus. They are a type of mucous retention cyst that forms in response to chronic maxillary sinusitis.

**Why the Correct Answer is Right**
The formation of Haller cells is a result of the mucous membrane's attempt to trap and sequester infected or inflamed mucous, leading to the formation of a cystic space within the infratemporal fossa. This process is often seen in patients with chronic maxillary sinusitis, where the mucous membranes are chronically inflamed and produce excess mucous.

**Why Each Wrong Option is Incorrect**
**Option A:** This option is incorrect because Haller cells are not typically seen in the frontal sinus, which is a different paranasal sinus with a distinct anatomy and pathology.
**Option B:** This option is incorrect because Haller cells are not typically seen in the sphenoid sinus, which is a posterior paranasal sinus with a different embryological origin and anatomy.
**Option C:** This option is incorrect because Haller cells are not typically seen in the ethmoid sinus, which is a group of small air cells that are often involved in chronic sinusitis, but not typically associated with the formation of Haller cells.

**Clinical Pearl / High-Yield Fact**
Haller cells are an important radiological finding in patients with chronic maxillary sinusitis, and their presence can be used to support a diagnosis of chronic sinusitis on imaging studies.

**Correct Answer: D. Haller cells are seen in the infratemporal fossa, associated with the maxillary sinus.**