The correct answer is A. S100.
This photo depicts a granular cell tumor. These typically present as asymptomatic skin-colored or brownish red, firm, dermal papules. They are usually solitary and have a predilection for the tongue (70%). Histologically, they present with round cells with distinctly granular cytoplasm. They stain S100+ and have PAS+ lysosomal inclusions (pustulo-ovoid bodies of Milian).
MART1 is a melanocyte marker that can be positive in nevi and melanoma. Von Kossa stains calcium, which would stain positive in calciphylaxis and Pseudoxanthoma elasticum. Giemsa stains mast cells and would be positive in a mastocytoma. Congo red stains amyloid and would be positive in amyloidosis.
Elston D, Ferringer T. Dermatopathology: Expert Consult, 2nd ed. Philadelphia: Elsevier Saunders; 2014.