**Core Concept**
The Eustachian tube, also known as the auditory tube, is a narrow tube that connects the middle ear to the back of the throat. Its primary function is to regulate air pressure in the middle ear and equalize atmospheric pressure with the environment.

**Why the Correct Answer is Right**
The Eustachian tube passes between the middle ear and the nasopharynx, which is the upper part of the throat behind the nasal cavity. This location allows the tube to regulate air pressure and maintain a functional patency of the middle ear. The Eustachian tube is innervated by the glossopharyngeal nerve (cranial nerve IX) and is lined with mucous membranes that produce mucus to help humidify and protect the middle ear.

**Why Each Wrong Option is Incorrect**
**Option A:** This option is incorrect because the Eustachian tube does not pass between the middle ear and the external auditory meatus (the ear canal).
**Option B:** This option is incorrect because the Eustachian tube does not pass between the middle ear and the mastoid air cells.
**Option C:** This option is incorrect because the Eustachian tube does not pass between the middle ear and the sphenoid sinus.

**Clinical Pearl / High-Yield Fact**
A key clinical correlation is that dysfunction of the Eustachian tube can lead to otitis media with effusion (fluid buildup in the middle ear), which is a common cause of hearing loss in children.

**Correct Answer:** C. The Eustachian tube passes between the middle ear and the nasopharynx.