**Core Concept**
The patient in question has a solitary kidney with a localized renal mass, necessitating a management approach that balances oncological efficacy with nephron-sparing techniques to preserve renal function.

**Why the Correct Answer is Right**
The presence of a solitary kidney warrants a conservative approach to renal preservation. Partial nephrectomy (PN) is the preferred treatment option for localized renal masses in patients with a solitary kidney, as it offers an equivalent oncological outcome to radical nephrectomy while preserving renal function. PN involves excising the tumor, preserving as much normal renal tissue as possible, and reconstructing the kidney.

**Why Each Wrong Option is Incorrect**
**Option A:** Radical nephrectomy is not the preferred option in a patient with a solitary kidney, as it would result in the loss of the only kidney, necessitating lifelong dialysis or renal transplantation.

**Option B:** Active surveillance is not suitable for a patient with a solitary kidney, as it would not address the underlying malignant process, and the consequences of delayed intervention could be severe.

**Option C:** Neoadjuvant chemotherapy or radiation therapy is not typically recommended as a primary treatment for localized renal masses, especially in a patient with a solitary kidney, where preserving renal function is a priority.

**Clinical Pearl / High-Yield Fact**
Partial nephrectomy should be performed by an experienced urologist or a nephrologist to minimize the risk of complications and ensure optimal oncological and renal function outcomes.

**Correct Answer:** C. Partial nephrectomy.