**Core Concept**
The parotid duct, also known as the Stensen's duct, is a **salivary duct** that plays a crucial role in the secretion of saliva from the **parotid gland**. The duct's opening is an essential anatomical landmark in the oral cavity.

**Why the Correct Answer is Right**
The parotid duct opens into the **oral cavity**, specifically into the **mouth**, to facilitate the secretion of saliva. This duct is approximately 6-7 cm long and opens near the second **molar tooth**. The correct answer is related to the anatomical location where the parotid duct empties its secretions.

**Why Each Wrong Option is Incorrect**
**Option A:** Incorrect because it is not the correct location.
**Option B:** Incorrect as it does not relate to the parotid duct's opening.
**Option C:** Incorrect because the parotid duct does not open here.

**Clinical Pearl / High-Yield Fact**
The parotid duct's opening is clinically significant, especially during **sialolithiasis** or **salivary gland infections**. Remembering the exact location of the duct's opening can aid in diagnosis and treatment.

**Correct Answer:** D. Mouth