**Core Concept**
The TNM staging system for colorectal cancer is a widely used classification system that helps predict the prognosis and guides treatment decisions. The staging system takes into account the size and extent of the primary tumor (T), the presence of lymph nodes involved (N), and the presence of distant metastasis (M).

**Why the Correct Answer is Right**
In the TNM staging system, stage III colon cancer is further divided into three subcategories: IIIa, IIIb, and IIIc. Stage IIIc is characterized by the presence of 4 or more positive lymph nodes. This indicates a more advanced disease with a higher risk of recurrence. The presence of a large number of involved lymph nodes suggests a higher tumor burden and a greater likelihood of microscopic metastases.

**Why Each Wrong Option is Incorrect**
**Option A:** Incorrect, as stage IIIa is characterized by 1-3 positive lymph nodes.
**Option B:** Incorrect, as stage IIIb is characterized by the presence of a perforated tumor and/or obstruction.
**Option C:** Incorrect, as stage IV is characterized by the presence of distant metastasis.

**Clinical Pearl / High-Yield Fact**
Remember that the number of positive lymph nodes is a critical factor in determining the stage of colon cancer. A higher number of involved lymph nodes is associated with a worse prognosis and may require more aggressive treatment.

**Correct Answer: D. Stage IIIc is characterized by the presence of 4 or more positive lymph nodes.**