**Core Concept**
The parotid duct, also known as the Stenson's duct, is a salivary duct that carries saliva from the parotid gland to the oral cavity. The location of the duct's opening is an important anatomical landmark.

**Why the Correct Answer is Right**
The parotid duct opens opposite the second upper molar tooth, specifically in the oral cavity. This is due to the anatomical relationship between the parotid gland and the surrounding structures. The duct pierces the buccinator muscle and opens in the oral cavity, allowing saliva to flow from the gland into the mouth. The precise location of the duct's opening is essential for dental and oral surgical procedures.

**Why Each Wrong Option is Incorrect**
* **Option A:** This option is incorrect because the parotid duct does not open opposite the first upper molar tooth. The anatomy of the parotid gland and its duct is such that the opening is located more distally in the oral cavity.
* **Option B:** This option is incorrect because the parotid duct does not open opposite the lower molars or premolars. The duct's opening is specifically located opposite the second upper molar tooth.
* **Option C:** This option is incorrect because the parotid duct does not open opposite the canine or incisors. The anatomy of the parotid gland and its duct is such that the opening is located more posteriorly in the oral cavity.

**Clinical Pearl / High-Yield Fact**
It's essential for dental and oral surgeons to be aware of the precise location of the parotid duct's opening to avoid damaging the duct during surgical procedures. The duct's opening can be identified by the presence of a small papilla in the oral mucosa opposite the second upper molar tooth.

**Correct Answer:** C.