Which is the MOST definitive characteristic that distinguishes a malignant tumor from a benign tumor?

The correct answer is d).

The question asks for the most definitive feature distinguishing a malignant tumor from a benign one.

• a) A high mitotic index indicates rapid cell division. While malignant tumors generally have a higher mitotic index than benign ones, some benign tumors can also have a relatively high mitotic index, and some malignant tumors can have a lower one. Therefore, it's not always definitive.
• b) The presence of a fibrous capsule is typically a feature of benign tumors, which are often well-demarcated and encapsulated. Malignant tumors are usually invasive and lack a well-defined capsule. So, this option describes a benign feature.
• c) A slow growth rate is characteristic of many benign tumors. Malignant tumors typically exhibit rapid, uncontrolled growth. So, this option describes a benign feature.
• d) Metastasis is the spread of cancer cells from the primary tumor to distant sites in the body, forming new tumors. This ability to invade surrounding tissues and spread to distant organs is the hallmark of malignancy and is never seen in benign tumors. Therefore, it is the most definitive feature.

The final answer is $\boxed{\text{d. Metastasis.}}$