**Core Concept:**
The question is asking about the most common primary malignant tumor of long bones in adults. Malignant tumors refer to cancerous cells that have the potential to invade nearby tissues and spread to other parts of the body. In long bones, the most common primary malignant tumor is Ewing's sarcoma, which accounts for about 10% of all childhood and adolescent bone tumors.

**Why the Correct Answer is Right:**
Ewing's sarcoma (ES) is a type of bone cancer that primarily affects long bones, particularly in children and young adults. This tumor is characterized by the overgrowth of primitive, undifferentiated neuroectodermal cells, which leads to the destruction of bone tissue and invasion into surrounding soft tissues. The tumor cells produce a specific fusion protein (EWS-FLI1) that drives the aggressive growth and spread of the cancer.

**Why Each Wrong Option is Incorrect:**
A. Osteosarcoma (OS) is the most common primary malignant tumor of long bones in adults, not children. It develops from mesenchymal stem cells and involves the production of osteoid, a substance that forms the bone matrix. While both OS and ES can affect long bones, the underlying cell types differ, making OS a more appropriate choice for this question.
B. Chondrosarcoma is a type of bone cancer that develops from the cartilage-forming cells and involves the production of cartilage matrix. This option is incorrect because it is less common than Ewing's sarcoma in children and adolescents.
C. Ewing's sarcoma is not as common in the axial skeleton (spine, pelvis, skull) compared to long bones. This option is incorrect due to the specific location preference for Ewing's sarcoma in long bones.
D. Rhabdomyosarcoma is a cancer that originates from embryonic muscle cells and is less common in long bones than Ewing's sarcoma.

**Clinical Pearl:**
Ewing's sarcoma is a pediatric malignancy with distinct clinical, radiological, and pathological features. Its aggressive nature and location preference make it the correct answer for this question, while the other options represent distinct cancer types with different characteristics and locations. It is essential to remember the age range, tumor location, and histopathological features of each tumor type to differentiate between them accurately.