**Core Concept**
Expansile lytic lesions with fluid-fluid levels within the metaphysis of long bones in children and adolescents are characteristic of a specific type of bone tumor. This phenomenon occurs due to the presence of fluid-filled spaces within the lesion, which is a hallmark of this particular condition.

**Why the Correct Answer is Right**
The correct answer is a type of bone tumor that arises from the cartilage cells (chondrocytes) and is known for its characteristic fluid-fluid levels on imaging studies. This occurs due to the presence of multiple fluid-filled cavities within the lesion, which are separated by a thin layer of cartilage. The tumor's location within the metaphysis of long bones, such as the fibula, is also typical of this condition. The fluid-fluid levels are a result of the density difference between the blood or fluid within the cavities and the surrounding bone tissue.

**Why Each Wrong Option is Incorrect**
**Option A:** This option is incorrect because the lesion described is not typical of a simple bone cyst, which usually presents as a unilocular lytic lesion without fluid-fluid levels.

**Option B:** This option is incorrect because osteosarcoma typically presents as a destructive lytic lesion with cortical breakthrough and soft tissue mass, rather than an expansile lesion with fluid-fluid levels.

**Option C:** This option is incorrect because Ewing's sarcoma typically presents as a destructive lytic lesion with cortical breakthrough and soft tissue mass, rather than an expansile lesion with fluid-fluid levels.

**Option D:** This option is incorrect because giant cell tumor typically presents as a lytic lesion in the epiphysis or apophysis of long bones, rather than the metaphysis.

**Clinical Pearl / High-Yield Fact**
The presence of fluid-fluid levels in a bone lesion is highly suggestive of a cartilaginous origin, such as chondroblastoma. This is a classic radiological feature that can help narrow down the differential diagnosis in cases of expansile lytic lesions in children and adolescents.

**Correct Answer:** C. Chondroblastoma.