**Core Concept**
The staging of oral cancer, including tongue carcinoma, depends on the size and extent of the primary tumor, lymph node involvement, and distant metastasis. The **TNM staging system** is used for this purpose, where T refers to the size and extent of the primary tumor, N refers to lymph node involvement, and M refers to distant metastasis.

**Why the Correct Answer is Right**
Given that the patient has carcinoma of the right tongue on its lateral border of the anterior 2/3rd with a lymph node of size 4 cm in level 3 on the left side of the neck, this indicates **contralateral lymph node metastasis**, which is a significant factor in staging. The size of the primary tumor and the presence of lymph nodes, especially in a contralateral location, suggest an advanced stage of the disease.

**Why Each Wrong Option is Incorrect**
**Option A:** This would typically represent an earlier stage of the disease without significant lymph node involvement or distant metastasis.
**Option B:** This might represent a stage with smaller lymph nodes or less extensive disease.
**Option C:** This could be a possibility but does not accurately reflect the given clinical scenario as precisely as the correct answer.

**Clinical Pearl / High-Yield Fact**
In oral cancers, including tongue carcinomas, the presence of **contralateral lymph nodes** or lymph nodes in the lower neck (Level 4) is associated with a worse prognosis and advanced disease stage. The **TNM staging system** is crucial for determining the prognosis and guiding treatment.

**Correct Answer:** D. Stage IV