**Core Concept**
The prognosis of cancer esophagus is influenced by several factors, but the most important one is the depth of tumor invasion into the esophageal wall. This is because the depth of invasion correlates with the likelihood of lymph node metastasis and distant spread.

**Why the Correct Answer is Right**
The depth of tumor invasion is a critical factor in determining the prognosis of esophageal cancer. According to the TNM staging system, the depth of invasion is classified into four categories: T1 (invasion into the mucosa), T2 (invasion into the submucosa), T3 (invasion into the muscularis propria), and T4 (invasion into adjacent structures). The deeper the tumor invades, the greater the likelihood of lymph node metastasis and distant spread. This is because the submucosa contains lymphatic vessels that can facilitate the spread of cancer cells to regional lymph nodes.

**Why Each Wrong Option is Incorrect**
**Option A:** Tumor grade is an important prognostic factor, but it is not as critical as the depth of invasion. Tumor grade refers to the degree of differentiation of cancer cells, with higher grades indicating more aggressive tumors.
**Option B:** Lymph node metastasis is a common feature of esophageal cancer, but it is not the most important prognostic factor. Lymph node metastasis can occur at any depth of tumor invasion, and the presence of lymph node metastasis is a sign of advanced disease.
**Option C:** The location of the tumor within the esophagus is not a critical prognostic factor. Tumors in the upper esophagus are not inherently more or less aggressive than tumors in the lower esophagus.

**Clinical Pearl / High-Yield Fact**
It is essential to remember that the depth of tumor invasion is a critical factor in determining the prognosis of esophageal cancer. A tumor that invades the submucosa (T2) has a significantly better prognosis than a tumor that invades the muscularis propria (T3) or adjacent structures (T4).

**Correct Answer:** C. Depth of tumor invasion.