**Core Concept**
Malignant transformation of uterine fibroids, also known as leiomyosarcoma, is a rare but serious complication of these benign tumors. The risk of malignancy is typically assessed in the context of clinical presentation and radiological findings.

**Why the Correct Answer is Right**
The risk of malignant transformation of fibroids is extremely low, estimated to be around 0.1-0.4% in most studies. This implies that the vast majority of fibroids are benign. However, certain factors such as large tumor size, rapid growth, and postmenopausal bleeding may raise suspicion for malignancy. Histopathological examination is the gold standard for diagnosing leiomyosarcoma, which typically shows nuclear atypia and high mitotic activity.

**Why Each Wrong Option is Incorrect**
**Option A:** This option is not provided as the correct answer is not listed.
**Option B:** There is no established correlation between fibroid malignancy and a specific age group.
**Option C:** While fibroid size and growth rate can be concerning, they do not directly correlate with malignancy risk.

**Clinical Pearl / High-Yield Fact**
Recall that the risk of malignancy in fibroids is extremely low, and most fibroids are benign. Clinical suspicion for malignancy should be based on a combination of factors, including rapid growth, postmenopausal bleeding, and atypical histopathological features.

**Correct Answer:** D. 0.1-0.4%