**Core Concept**
The size of an ovarian tumor is a critical factor in determining its malignant potential. Ovarian tumors can be benign or malignant, and their size is one of the key criteria used to differentiate between them. The American Joint Committee on Cancer (AJCC) and the International Federation of Gynecology and Obstetrics (FIGO) have established size criteria for ovarian tumors to predict their malignant potential.

**Why the Correct Answer is Right**
A tumor size of 12 cm or more is generally considered to be malignant. This is because larger tumors have a higher likelihood of being cancerous and have a greater potential to spread to other parts of the body. The size criterion is based on the observation that larger tumors have a higher mitotic rate, more chromosomal abnormalities, and a greater potential for invasion and metastasis. The size of the tumor is an important factor in determining the prognosis and treatment of ovarian cancer.

**Why Each Wrong Option is Incorrect**
**Option A:** This option is incorrect because a tumor size of 8 cm is not considered to be definitely malignant. While it is possible for tumors of this size to be cancerous, a size of 8 cm is not a reliable indicator of malignancy.
**Option B:** This option is incorrect because a tumor size of 10 cm is not universally accepted as a threshold for malignancy. While larger tumors are more likely to be cancerous, a size of 10 cm is not a definitive indicator of malignancy.
**Option C:** This option is incorrect because a tumor size of 15 cm is not a more reliable indicator of malignancy than a size of 12 cm. In fact, a tumor size of 15 cm is likely to be more aggressive and have a poorer prognosis than a tumor of 12 cm.

**Clinical Pearl / High-Yield Fact**
Remember the "SOB" rule for ovarian tumors: Size, Omentum, and Bilaterality. Tumors that are large (> 12 cm), involve the omentum, or are bilateral are more likely to be malignant.

**Correct Answer: C. 12 cm or more.**