**Core Concept**
The parotid gland is a major salivary gland that secretes serous saliva into the oral cavity. The parotid duct, also known as Stensen's duct, is a critical anatomical structure that facilitates the drainage of saliva from the gland into the mouth.

**Why the Correct Answer is Right**
The parotid duct opens into the oral cavity, specifically into the buccal mucosa opposite the upper second molar tooth. This location allows the saliva to mix with food and aid in digestion. The duct is about 5 cm long and is situated anterior to the masseter muscle and posterior to the zygomatic arch.

**Why Each Wrong Option is Incorrect**
* **Option A:** This option is incorrect because the parotid duct does not open into the floor of the mouth. The submandibular gland, not the parotid gland, drains into the floor of the mouth via the submandibular duct (Wharton's duct).
* **Option B:** This option is incorrect because the parotid duct does not open into the nasal cavity. The nasal cavity receives secretions from the nasal mucosa and the anterior ethmoidal sinus, but not from the parotid gland.
* **Option C:** This option is incorrect because the parotid duct does not open into the pharynx. The parotid gland does not drain into the pharynx; instead, it opens into the oral cavity to facilitate digestion.

**Clinical Pearl / High-Yield Fact**
The parotid duct can be palpated during a physical examination, and its opening can be identified by gently pressing the buccal mucosa opposite the upper second molar tooth.

**Correct Answer:** C.