**Core Concept**
Stensen's duct, also known as the parotid duct, is a salivary gland duct that conveys saliva from the parotid gland to the oral cavity. The correct location of its opening is a key anatomical detail for dental and surgical procedures.

**Why the Correct Answer is Right**
The parotid gland is a major salivary gland located in the preauricular region of the face. Stensen's duct opens into the oral cavity opposite the upper second molar, allowing saliva to flow into the mouth and facilitate digestion. This location allows for efficient mixing of saliva with food during mastication. The precise opening of Stensen's duct is also relevant for the diagnosis and treatment of parotid gland disorders, such as sialolithiasis and parotid tumors.

**Why Each Wrong Option is Incorrect**
**Option A:** Incorrect because the opening of Stensen's duct is not opposite the lower second molar. This option might be tempting due to the proximity of the lower second molar to the parotid gland, but anatomically, the duct opens higher up in the oral cavity.
**Option B:** Incorrect because the opening of Stensen's duct is not opposite the upper first molar. This option might be chosen due to the proximity of the upper first molar to the parotid gland, but the duct opens more posteriorly in the oral cavity.
**Option C:** Incorrect because the opening of Stensen's duct is not opposite the lower first molar. This option might be chosen due to the proximity of the lower first molar to the parotid gland, but the duct opens higher up in the oral cavity.

**Clinical Pearl / High-Yield Fact**
A key clinical correlation is that the opening of Stensen's duct is often used as a landmark for the location of the parotid gland. Knowing the precise location of the duct's opening can help clinicians diagnose and manage parotid gland disorders, such as sialolithiasis and parotid tumors.

**Correct Answer: D. Opposite to the upper second molar.**