**Core Concept**
The staging of oral cancer, including carcinoma of the tongue, is based on the size and extent of the primary tumor (T), the presence and extent of lymph node metastasis (N), and the presence of distant metastasis (M). The **TNM staging system** is used to classify the disease.

**Why the Correct Answer is Right**
Given the patient has a carcinoma on the lateral border of the anterior 2/3rd of the right tongue and a lymph node of size 4 cm in level 3 on the **contralateral** side of the neck, this indicates advanced disease with significant lymph node involvement. The size and location of the lymph node, as well as its presence on the opposite side of the primary tumor, are critical for determining the N stage, which in turn affects the overall stage of the disease.

**Why Each Wrong Option is Incorrect**
**Option A:** This would be incorrect because it typically represents an earlier stage of cancer without significant lymph node involvement or distant metastasis.
**Option B:** Similarly, this option would not accurately reflect the stage given the significant lymph node involvement on the contralateral side.
**Option D:** This option would also be incorrect as it does not match the provided clinical scenario of lymph node involvement on the opposite side of the primary tumor.

**Clinical Pearl / High-Yield Fact**
It's crucial to remember that the **TNM staging system** is dynamic and can change based on the specifics of the tumor and its spread. For oral cancers, including tongue carcinoma, the presence of lymph nodes, especially those that are large or located on the opposite side of the primary tumor, significantly impacts staging and treatment planning.

**Correct Answer:** C. T2N2cM0