**Core Concept**
The prognosis of mesenchymal tumors, which arise from connective tissue, is influenced by several factors, including histological grade, tumor size, and mitotic rate. **Tumor grade** is a key factor as it reflects the degree of cellular differentiation and proliferation.

**Why the Correct Answer is Right**
The correct answer is related to the **histological grade**, as it is the most important prognostic indicator for mesenchymal tumors. A higher grade indicates a more aggressive tumor with a poorer prognosis. This is because high-grade tumors have more cellular atypia, increased mitotic activity, and often, a greater potential for metastasis.

**Why Each Wrong Option is Incorrect**
**Option A:** Incorrect because while tumor size can influence prognosis, it is not the most important factor.
**Option B:** Incorrect as tumor location can affect treatment options but is not the primary prognostic indicator.
**Option D:** Incorrect because although mitotic rate is a component of tumor grade, it is not the sole most important prognostic indicator on its own.

**Clinical Pearl / High-Yield Fact**
A key point to remember is that the **histological grade** of a tumor is crucial for determining prognosis and guiding treatment decisions in mesenchymal tumors.

**Correct Answer:** D. Histological grade.