**Core Concept**
The nasal septum is a midline structure within the nasal cavity, composed of cartilage and bone. It is a key landmark in radiological imaging of the paranasal sinuses and nasal passages. The nasal septum's radiopacity is due to the presence of bone and cartilage, which are denser than surrounding soft tissues.

**Why the Correct Answer is Right**
The nasal septum's radiopacity is a result of its bony and cartilaginous composition. The perpendicular plate of the ethmoid bone and the vomer bone form the posterior and inferior aspects of the nasal septum, respectively. These bony components contribute to the septum's radiopacity. Additionally, the septal cartilage, composed of hyaline cartilage, also contributes to the structure's visibility on radiographs. The combination of bone and cartilage makes the nasal septum appear more radiopaque compared to surrounding soft tissues.

**Why Each Wrong Option is Incorrect**
**Option A:** The maxillary sinus is a paranasal air-filled cavity that appears radiolucent due to its air-filled nature. It is not radiopaque.

**Option B:** The nasal fossa refers to the nasal cavity, which is composed of soft tissues and appears radiolucent on radiographs.

**Option D:** The mental foramen is a small opening in the mandible through which the mental nerve and vessels pass. It is not a significant radiopaque structure in the nasal cavity.

**Clinical Pearl / High-Yield Fact**
It is essential to recognize the radiopacity of the nasal septum on radiographs to accurately diagnose nasal septal deviations or other nasal pathologies. A well-positioned nasal septum is often a key landmark in paranasal sinus imaging.

**✓ Correct Answer: C. Nasal septum**