**Core Concept**
The question is testing the student's knowledge of common soft tissue tumors in children. Soft tissue tumors are a broad category of neoplasms that arise from mesenchymal cells, which include connective tissue, fat, blood vessels, and muscle. In children, the most common soft tissue tumor is typically a type of benign growth.

**Why the Correct Answer is Right**
The most common soft tissue tumor in children is a benign growth known as a **hemangioma**. Hemangiomas are vascular tumors that consist of an abnormal proliferation of blood vessels. They are often present at birth or appear in the first few weeks of life and typically resolve on their own by the age of 5-7. Hemangiomas are more common in girls and are often found on the skin, but they can also be found in other soft tissues.

**Why Each Wrong Option is Incorrect**
* **Option A:** This is incorrect because **lipoma**, a benign tumor of fat tissue, is less common in children compared to hemangiomas.
* **Option B:** This is incorrect because **rhabdomyoma**, a benign tumor of smooth muscle, is rare in children and more often associated with cardiac or genital malformations.
* **Option C:** This is incorrect because **fibroma**, a benign tumor of fibrous tissue, is less common in children compared to hemangiomas.

**Clinical Pearl / High-Yield Fact**
It is essential for clinicians to recognize the distinction between hemangiomas and other vascular tumors, such as **vascular malformations**, which are present at birth and do not regress with age.

**Correct Answer:** D. Hemangioma