**Core Concept**
The question is testing the ability to identify a specific radiological finding associated with a particular condition. Lytic lesions are areas of bone destruction, and an expansive lytic lesion in the transverse process of a vertebra suggests a condition that causes localized bone destruction.

**Why the Correct Answer is Right**
The correct answer is Giant Cell Tumor (GCT). GCT is a type of bone tumor that typically affects the epiphyseal ends of long bones, but it can also occur in the vertebrae. It is characterized by an expansive lytic lesion with a thin layer of cortical bone surrounding the tumor. The transverse process of a vertebra is a common site for GCT. The tumor is composed of giant cells, mononuclear stromal cells, and osteoclast-like cells, which are responsible for the bone destruction.

**Why Each Wrong Option is Incorrect**
* **Option A:** Osteolytic metastases from a primary tumor such as prostate cancer can cause lytic lesions in the bone, but they are typically more aggressive and destructive than GCT, with a lack of a distinct boundary between the tumor and the surrounding bone.
* **Option B:** Brown tumor of hyperparathyroidism is a lytic lesion caused by excessive parathyroid hormone, which leads to bone resorption. However, it typically affects the flat bones of the skull, ribs, and pelvis, rather than the transverse process of a vertebra.
* **Option D:** Osteoblastoma is a rare, benign tumor that can cause lytic lesions, but it typically affects the long bones of the limbs, and is characterized by a mixed lytic and sclerotic appearance on radiographs.

**Clinical Pearl / High-Yield Fact**
Giant Cell Tumor is a common cause of lytic lesions in the vertebrae, and a classic example of a "soap bubble" appearance on radiographs. It is a benign tumor, but it can cause significant bone destruction and pain, and may require surgical treatment.

**Correct Answer:** C. Giant Cell Tumor.