**Core Concept**
The Eustachian tube is a critical structure that connects the middle ear to the nasopharynx, allowing for the equalization of air pressure on both sides of the eardrum. This tube is essential for the proper functioning of the middle ear, facilitating drainage of mucus and maintaining aeration.

**Why the Correct Answer is Right**
The Eustachian tube passes between the middle ear and the nasopharynx, specifically through the cartilaginous part of the auditory tube, which is a short, tube-like structure that joins the middle ear to the nasopharynx. The Eustachian tube opens into the nasopharynx via the torus tubarius, a bony projection that forms the lateral wall of the nasopharynx. This opening allows for the exchange of air and mucus between the middle ear and the nasopharynx.

**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 mastoid air cells. While the mastoid air cells are air-filled spaces in the temporal bone, they are not directly connected to the Eustachian tube.
* **Option B:** This option is incorrect because the Eustachian tube does not pass between the middle ear and the external auditory meatus. The external auditory meatus, or ear canal, is the external passage that leads to the eardrum, but it is not directly connected to the Eustachian tube.
* **Option C:** This option is incorrect because the Eustachian tube does not pass between the middle ear and the oral cavity. While the oral cavity is the cavity that contains the mouth, it is not directly connected to the Eustachian tube.

**Clinical Pearl / High-Yield Fact**
It's essential to remember that dysfunction of the Eustachian tube can lead to middle ear problems, such as otitis media with effusion (fluid accumulation in the middle ear). This condition can result from Eustachian tube dysfunction, which can be caused by various factors, including allergies, infections, or anatomical abnormalities.

**Correct Answer: D**