**Core Concept**
The question is testing the understanding of the TNM staging system for oral cavity cancers, specifically the classification of the primary tumor (T) and the regional lymph nodes (N) based on their size and extent.

**Why the Correct Answer is Right**
The TNM staging system is a widely used classification system for cancer staging. For oral cavity cancers, the T category is based on the size and extent of the primary tumor. A 3cm tumor is classified as T2, indicating a tumor that is more than 2cm but not more than 4cm in greatest dimension. The N category is based on the size and number of regional lymph nodes involved. A single lymph node of 2cm diameter on the same side in the neck is classified as N1, indicating movable lymph nodes with a maximum diameter of 3cm. Since there are no distant metastases, the M category is M0.

**Why Each Wrong Option is Incorrect**
**Option A:** This option is incorrect because it does not take into account the size of the lymph node and the absence of distant metastases.
**Option B:** This option is incorrect because it is not a valid classification for the primary tumor and regional lymph nodes.
**Option C:** This option is incorrect because it is not a valid classification for the primary tumor and regional lymph nodes.

**Clinical Pearl / High-Yield Fact**
Remember that the TNM staging system is used to predict prognosis and guide treatment decisions for oral cavity cancers. The classification of the primary tumor and regional lymph nodes is critical in determining the stage and treatment plan.

**Correct Answer:** T2 N1 M0