Stain used in electron microscopy: (A) 2.5% FAM, (B) Phosphotungstic acid, (C) Saffranin, (D) Coomassie blue
Answer is B. Phosphotungstic acid is electron dense, opaque for electrons. It is a common negative stain for viruses, nerves, polysaccharides and other biological tissue materials for imaging by an Electron microscope.