**Core Concept**
Cetuximab is a monoclonal antibody targeting the epidermal growth factor receptor (EGFR), a transmembrane receptor involved in cell proliferation and survival. By binding to EGFR, cetuximab prevents ligand-induced receptor dimerization, thereby inhibiting downstream signaling pathways that promote tumor growth.

**Why the Correct Answer is Right**
Cetuximab is used in the treatment of colorectal cancer, specifically in patients with metastatic disease or those who have failed previous therapies. This is because EGFR overexpression is a common feature in colorectal cancer, and cetuximab's mechanism of action can help to inhibit tumor growth and induce apoptosis. The addition of cetuximab to chemotherapy has been shown to improve survival outcomes in this patient population.

**Why Each Wrong Option is Incorrect**
**Option A:** Cetuximab is not typically used in the treatment of breast cancer, which often involves different molecular pathways and targets.
**Option B:** While cetuximab does target EGFR, it is not primarily used in the treatment of lung cancer, where other agents such as tyrosine kinase inhibitors may be more effective.
**Option C:** Cetuximab is not used in the treatment of gastric cancer, which often involves different molecular pathways and targets.

**Clinical Pearl / High-Yield Fact**
When considering the use of cetuximab or other EGFR antagonists, it's essential to assess for EGFR expression in the tumor, as this can impact treatment efficacy and toxicity.

**Correct Answer:** C. Colorectal cancer.