You are answering a USMLE-style medical question. Question: A 60-year-old woman presents to a physician for worsening shortness of breath and increasing abdominal distention over the last 3 months. She says that the shortness of breath is worse on exertion and improves with rest. While she could previously walk to the nearby store for her groceries, she now has to drive because she gets ''winded'' on the way. The patient was diagnosed with diabetes 5 years ago and is compliant with her medications. The medical history is otherwise unremarkable. The physical examination reveals gross ascites and visibly engorged periumbilical veins. Bilateral pitting edema is noted around the ankles. The finger-prick blood glucose level is 100 mg/dL. What is the mechanism of action of the anti-diabetic medication this patient is most likely taking? Options: A) Binding to the alpha subunit of the insulin receptor B) Closure of ATP-sensitive K-channels in the pancreatic beta-cell C) Increased gene expression of GLUT-4 D) Glucagon-like peptide-1 receptor agonist Reply with ONLY the letter of the correct answer (A, B, C, or D). No explanation.