**Core Concept**
Gastrointestinal stromal tumors (GISTs) are the most common mesenchymal neoplasms of the gastrointestinal tract, often arising in the stomach. They are characterized by their spindle cell morphology and positivity for the c-Kit protein, making them responsive to tyrosine kinase inhibitors.

**Why the Correct Answer is Right**
The patient's presentation of a large gastric mass with spindle cells positive for c-Kit on immunohistochemical staining is consistent with a GIST. Given the high mitotic rate, the tumor is likely to be malignant. The most effective treatment for GISTs is targeted therapy with tyrosine kinase inhibitors, such as imatinib, which specifically inhibit the c-Kit protein, thereby blocking tumor growth.

**Why Each Wrong Option is Incorrect**
* **Option A:** Chemotherapy is not the primary treatment for GISTs, as they are not responsive to traditional cytotoxic agents.
* **Option B:** Radiation therapy may be used as a palliative measure to control symptoms, but it is not a standard treatment for GISTs.
* **Option D:** Surgery is indeed a critical component of GIST management, but the question asks for an adjunct therapy, making this option incomplete.

**Clinical Pearl / High-Yield Fact**
GISTs are often misdiagnosed as gastric adenocarcinomas, highlighting the importance of immunohistochemical staining for c-Kit in distinguishing between these two entities.

**Correct Answer:** C. Imatinib.