**Core Concept**
The question is testing knowledge of the most common type of sarcoma in children, which is a malignant tumor derived from mesenchymal cells. Sarcomas are rare and diverse, with different subtypes affecting various tissues and age groups.

**Why the Correct Answer is Right**
The correct answer is **Rhabdomyosarcoma** (RMS). RMS is a type of soft tissue sarcoma that arises from skeletal muscle cells. It is the most common type of sarcoma in children, accounting for approximately 50% of all pediatric sarcomas. RMS often presents with symptoms such as a painless mass, swelling, or bleeding. The exact cause of RMS is unknown, but genetic mutations play a significant role in its development.

**Why Each Wrong Option is Incorrect**
* **Option A:** Ewing's Sarcoma is a type of bone cancer, not a soft tissue sarcoma, and is more common in adolescents and young adults.
* **Option B:** Osteosarcoma is a type of bone cancer, typically affecting the long bones, and is more common in teenagers and young adults.
* **Option C:** Synovial Sarcoma is a type of soft tissue sarcoma, but it is more common in adults and typically arises near joints.

**Clinical Pearl / High-Yield Fact**
RMS is often classified into two main subtypes: embryonal RMS (most common in children under 6 years) and alveolar RMS (more common in older children and adolescents).

**Correct Answer:** C. Rhabdomyosarcoma