**Core Concept**
The prognosis of soft tissue tumors is determined by the combination of histological grade, tumor size, depth, and the presence of metastatic disease. Histological grade, in particular, is a critical factor in predicting the behavior and outcome of these tumors.

**Why the Correct Answer is Right**
The histological grade of a soft tissue tumor is a reflection of its cellular proliferative activity, which is assessed through the evaluation of nuclear features, mitotic activity, and the presence of tumor necrosis. This grade is often categorized as low, intermediate, or high, with higher grades indicating a more aggressive tumor behavior. The depth and size of the tumor are also important prognostic factors, as deeper and larger tumors are more likely to recur and metastasize. The presence of metastatic disease at diagnosis is a clear indication of a poor prognosis.

**Why Each Wrong Option is Incorrect**
**Option A:** This option is incorrect because while tumor size is an important prognostic factor, it is not the sole determinant of prognosis in soft tissue tumors.
**Option B:** This option is incorrect because the histological grade of a tumor is a more reliable predictor of prognosis than the tumor's cellular origin.
**Option C:** This option is incorrect because while the presence of metastatic disease is an important prognostic factor, it is not the only factor that determines prognosis in soft tissue tumors.

**Clinical Pearl / High-Yield Fact**
When evaluating the prognosis of a soft tissue tumor, it is essential to consider the histological grade, tumor size, and depth, as these factors have a significant impact on treatment decisions and patient outcomes.

**Correct Answer: C. The presence of metastatic disease.**