**Core Concept**
The soap bubble appearance on X-ray is a characteristic radiographic feature of certain bone tumors, which is caused by the presence of multiple, well-defined, lytic lesions within the bone. This appearance is typically seen in tumors with a high degree of osteolysis and minimal bone reaction.

**Why the Correct Answer is Right**
The soap bubble appearance on X-ray is most commonly associated with **Multiple Myeloma**. This is due to the fact that multiple myeloma is a malignant proliferation of plasma cells within the bone marrow, which leads to the formation of multiple lytic lesions throughout the bone. These lesions are usually well-defined and have a characteristic "soap bubble" appearance on X-ray. The lack of significant bone reaction or sclerosis is also a hallmark of multiple myeloma, which contributes to the characteristic radiographic appearance.

**Why Each Wrong Option is Incorrect**
* **Option A:** This option is incorrect because **Osteosarcoma** typically presents with a different radiographic appearance, including a "sunburst" pattern or Codman's triangle due to the aggressive production of bone by the tumor cells.
* **Option B:** This option is incorrect because **Ewing's Sarcoma** also presents with a different radiographic appearance, including a "moth-eaten" or "onion-skin" pattern due to the aggressive destruction of bone by the tumor cells.
* **Option C:** This option is incorrect because **Chondrosarcoma** typically presents with a different radiographic appearance, including a "rings and arcs" pattern due to the presence of calcified cartilage within the tumor.

**Clinical Pearl / High-Yield Fact**
Remember that the soap bubble appearance on X-ray is a classic radiographic feature of multiple myeloma, but it can also be seen in other conditions such as metastatic disease, particularly from breast or lung cancer. A high index of suspicion and correlation with clinical findings is essential for making an accurate diagnosis.

**Correct Answer:** D.