**Core Concept**
The question is testing the student's knowledge of the most common primary malignant tumor affecting the skeletal system. The skeletal system is a complex network of bones, joints, and associated tissues that provide structural support, facilitate movement, and protect internal organs.

**Why the Correct Answer is Right**
Multiple myeloma is the most common primary malignant tumor of the skeletal system. It is a type of plasma cell malignancy characterized by the proliferation of malignant plasma cells in the bone marrow, leading to bone destruction and marrow replacement. The disease is often associated with the production of abnormal monoclonal proteins, which can cause hypercalcemia, anemia, and renal impairment. The pathophysiology of multiple myeloma involves the interaction of malignant plasma cells with the bone microenvironment, leading to the activation of osteoclasts and the suppression of osteoblasts, resulting in bone resorption.

**Why Each Wrong Option is Incorrect**
**Option A:** Osteosarcoma is a type of primary bone cancer, but it is less common than multiple myeloma. Osteosarcoma arises from osteoblasts and is often associated with a worse prognosis than multiple myeloma.
**Option B:** Ewing's sarcoma is a rare type of primary bone cancer that typically affects children and young adults. While it is a malignant tumor, it is not as common as multiple myeloma in the skeletal system.
**Option C:** Chondrosarcoma is a type of primary bone cancer that arises from cartilage-producing cells. It is less common than multiple myeloma and often presents in older adults.

**Clinical Pearl / High-Yield Fact**
The diagnosis of multiple myeloma often involves a combination of imaging studies, bone marrow biopsy, and laboratory tests, including serum protein electrophoresis and immunofixation. A characteristic feature of multiple myeloma is the presence of lytic bone lesions, which can be detected on radiographs or CT scans.

**Correct Answer: D. Multiple myeloma**