**Core Concept**
The Eustachian tube connects the middle ear to the nasopharynx and plays a crucial role in equalizing the pressure on both sides of the eardrum. Its opening and closing are regulated by a complex mechanism involving muscles and ligaments.

**Why the Correct Answer is Right**
The Eustachian tube opens due to the contraction of the tensor veli palatini muscle. This muscle is innervated by the mandibular division of the trigeminal nerve (CN V3) and is responsible for tensing the tensor tympani muscle, which helps to regulate sound pressure. When the tensor veli palatini muscle contracts, it pulls on the cartilaginous portion of the Eustachian tube, causing it to open and allow air to enter the middle ear. This process is essential for equalizing the pressure in the middle ear with the atmospheric pressure.

**Why Each Wrong Option is Incorrect**
**Option A:** The levator veli palatini muscle is involved in elevating the soft palate during swallowing and speech, but it does not contribute to the opening of the Eustachian tube.
**Option B:** The palatoglossus muscle is involved in elevating the tongue and forming the anterior part of the palatine arch, but it does not play a role in regulating the Eustachian tube.
**Option C:** The salpingopharyngeus muscle is involved in elevating the pharynx during swallowing, but it does not contribute to the opening of the Eustachian tube.

**Clinical Pearl / High-Yield Fact**
The Eustachian tube is normally closed, and its opening is a critical aspect of maintaining middle ear pressure. Dysfunction of the Eustachian tube can lead to conditions such as otitis media with effusion (OME) and hearing loss.

**Correct Answer:** C. Tensor veli palatini muscle.