**Core Concept**
The soap bubble appearance on X-ray is a characteristic radiological feature of certain bone tumors, resulting from the presence of multiple, well-defined lytic lesions. This appearance is typically associated with bone lesions that involve the destruction of cortical bone and the formation of multiple radiolucent areas.

**Why the Correct Answer is Right**
The soap bubble appearance is classically seen in **Giant Cell Tumor (GCT)**, which is a type of benign bone tumor that typically affects the epiphyseal regions of long bones. The tumor consists of multinucleated giant cells and mononuclear stromal cells, leading to the characteristic lytic lesions. The soap bubble appearance is due to the presence of multiple, well-defined lytic lesions that give a "bubble-like" appearance on X-ray. The lesions are often located in the metaphyseal regions of long bones, such as the distal femur or proximal tibia.

**Why Each Wrong Option is Incorrect**
**Option A:** This option is incorrect because **Osteosarcoma** typically presents with a mixed lytic and sclerotic appearance on X-ray, rather than a soap bubble appearance. Osteosarcoma is a malignant bone tumor that involves the production of abnormal bone matrix.
**Option B:** This option is incorrect because **Multiple Myeloma** typically presents with a "punched-out" appearance on X-ray, rather than a soap bubble appearance. Multiple myeloma is a type of plasma cell dyscrasia that involves the proliferation of malignant plasma cells in the bone marrow.
**Option C:** This option is incorrect because **Ewing's Sarcoma** typically presents with a "moth-eaten" appearance on X-ray, rather than a soap bubble appearance. Ewing's sarcoma is a malignant bone tumor that involves the proliferation of primitive neuroectodermal cells in the bone.

**Clinical Pearl / High-Yield Fact**
A key feature of Giant Cell Tumor is its tendency to recur after surgical excision, due to the presence of small foci of tumor cells that may be left behind during surgery.

**Correct Answer:** C. Ewing's Sarcoma