**Core Concept**
The pathologist will use histopathological criteria to differentiate between benign and malignant uterine fibroids, also known as leiomyomas. This involves evaluating the tumor's cellular and architectural features.

**Why the Correct Answer is Right**
The pathologist will assess the tumor's mitotic rate, which is the number of mitotic figures (indicative of cell division) per 10 high-power fields (HPFs). A higher mitotic rate is associated with a greater likelihood of malignancy. Additionally, the pathologist will evaluate the tumor's cellular atypia, which refers to the degree of abnormality in cell shape and size. A higher degree of cellular atypia is also associated with malignancy. The pathologist may also use other criteria, such as tumor size, margin status, and presence of necrosis, to support the diagnosis.

**Why Each Wrong Option is Incorrect**
* **Option A:** While the pathologist may evaluate the tumor's cellular atypia, it is not the primary criterion used to determine benignancy versus malignancy.
* **Option B:** Tumor size is not a reliable criterion for determining malignancy in uterine fibroids.
* **Option D:** Presence of necrosis may be seen in both benign and malignant tumors and is not a specific criterion for malignancy.

**Clinical Pearl / High-Yield Fact**
The mitotic rate is a critical criterion in differentiating between benign and malignant uterine fibroids. A mitotic rate of 10 or more per 10 HPFs is often considered suspicious for malignancy.

**Correct Answer: C**