The answer is B. B. TTF-1.
The pictured image is that of merkel cell carcinoma (MCC), a rare highly aggressive malignant neuroendocrine carcinoma. In order to differentiate MCC from small cell lung carcinoma (SCLC; aka Oat cell carcinoma), another malignant neuroendocrine carcinoma, histologic staining with TTF-1 is most helpful. This is because MCC (nearly always) stains negative for TTF-1, whereas SCLC stains positive for TTF-1. The remaining answer choices will be positive in both MCC and SCLC, so they cannot be used to differentiate between the two malignancies.