**Core Concept**
The Eustachian tube is a vital anatomical structure responsible for equalizing the pressure between the middle ear and the atmosphere. It is a mucosa-lined tube that connects the middle ear to the nasopharynx, allowing for the exchange of air and maintaining ear pressure during changes in altitude or swallowing.

**Why the Correct Answer is Right**
The Eustachian tube passes between the cartilaginous part of the external auditory meatus and the bony part of the lateral wall of the nasopharynx. This unique location enables the tube to open into the nasopharynx, facilitating the drainage of mucus and the exchange of air. The Eustachian tube is innervated by the glossopharyngeal nerve (cranial nerve IX), which regulates its function.

**Why Each Wrong Option is Incorrect**
**Option A:** The Eustachian tube does not pass between the external auditory meatus and the petrous part of the temporal bone, as this would not allow for the exchange of air with the nasopharynx.
**Option B:** The Eustachian tube is not located between the tympanic membrane and the mastoid process, as this would not provide a direct connection to the nasopharynx.
**Option C:** The Eustachian tube is not situated between the external auditory meatus and the styloid process, as this would not facilitate the drainage of mucus into the nasopharynx.

**Clinical Pearl / High-Yield Fact**
The Eustachian tube is often affected in conditions such as otitis media with effusion, where the tube's dysfunction leads to the accumulation of fluid in the middle ear. It's essential to recognize the role of the Eustachian tube in ear infections and to consider its dysfunction in the diagnosis and management of middle ear disorders.

**Correct Answer: C. Between the cartilaginous part of the external auditory meatus and the bony part of the lateral wall of the nasopharynx.**