**Core Concept**
The patient's presentation with a lytic eccentric lesion in the lower end of the radius and a soap bubble appearance on X-ray is characteristic of a Giant Cell Tumor (GCT). GCTs are benign but locally aggressive bone tumors that typically occur in the epiphyseal regions of long bones, especially in skeletally mature individuals.

**Why the Correct Answer is Right**
GCTs are known for their lytic, eccentric lesions with a soap bubble appearance on X-ray due to the presence of multiple small cystic spaces. The correct management of GCT involves surgical excision, often with curettage, to remove the tumor tissue and prevent recurrence. The goal of surgery is to remove the tumor in its entirety while preserving the surrounding bone and joint structures.

**Why Each Wrong Option is Incorrect**
* **Option A:** This option is incorrect because it does not address the need for surgical intervention in the management of GCT.
* **Option B:** This option is incorrect because it implies a more conservative approach, such as observation or radiation therapy, which is not typically recommended for GCTs.
* **Option D:** This option is incorrect because it suggests a biopsy as the next step, which may not be necessary given the characteristic radiographic appearance of GCT.

**Clinical Pearl / High-Yield Fact**
Giant Cell Tumors are more likely to recur than other benign bone tumors, with a reported recurrence rate of up to 30%. Therefore, it is essential to employ a thorough surgical technique and follow up with regular radiographic monitoring to detect any potential recurrence.

**Correct Answer: C. Surgical excision with curettage.**