**Core Concept**
The parotid duct, also known as Stensen's duct, is a small tube that carries saliva from the parotid gland to the oral cavity. It is a crucial anatomical structure that plays a key role in the salivary gland system.

**Why the Correct Answer is Right**
The parotid duct opens into the oral cavity at the level of the second upper molar, also known as the first molar of the maxillary arch. This is a critical anatomical landmark for dental and surgical procedures. The duct is located on the cheek, and its opening is often visible as a small papilla. The parotid duct is formed by the duct of the parotid gland, which is the largest of the three major salivary glands. The duct is approximately 5 cm long and is innervated by the auriculotemporal nerve.

**Why Each Wrong Option is Incorrect**
**Option A:** The first premolar (PM) is not the correct location for the opening of the parotid duct. While the premolars are located in the same quadrant as the second upper molar, they are not the specific landmark for the duct's opening.
**Option B:** The first molar is actually the tooth adjacent to the second upper molar, and it is not the correct location for the parotid duct's opening.
**Option D:** The second lower molar is located in a different quadrant of the oral cavity and is not the correct location for the parotid duct's opening.

**Clinical Pearl / High-Yield Fact**
It is essential to note that the parotid duct's opening is often visible as a small papilla on the cheek, and its location can be used as a landmark for dental and surgical procedures. Knowing the correct location of the parotid duct's opening is crucial for avoiding damage to the duct during surgical procedures.

**✓ Correct Answer: C. 2nd upper molar**