**Core Concept**
Non-suppurative otitis media is a type of ear infection characterized by inflammation of the middle ear without the presence of pus. In adults, this condition is often caused by an abnormal Eustachian tube function, leading to negative middle ear pressure and subsequent inflammation.

**Why the Correct Answer is Right**
The correct answer is related to the Eustachian tube's role in regulating middle ear pressure. The Eustachian tube connects the middle ear to the nasopharynx and plays a crucial role in equalizing the pressure between the middle ear and the atmosphere. Abnormalities in the Eustachian tube's function, such as its patency or muscle tone, can lead to negative middle ear pressure, which in turn causes inflammation and non-suppurative otitis media. This condition is often associated with the nasopharyngeal carcinoma, which can obstruct the Eustachian tube.

**Why Each Wrong Option is Incorrect**
**Option A:** Eustachian tube obstruction is not typically caused by a foreign body.
**Option B:** While nasopharyngeal carcinoma can cause Eustachian tube obstruction, it is not the most common cause of non-suppurative otitis media in adults.
**Option C:** Middle ear infections are not typically caused by viral infections in adults.

**Clinical Pearl / High-Yield Fact**
The Eustachian tube's dysfunction can be caused by a variety of factors, including nasopharyngeal carcinoma, Eustachian tube dysfunction syndrome, and other conditions that affect the nasopharynx.

**Correct Answer:** C.