• Sensitivity of Protein Stains in Polyacrylamide Gels (PAGE):
• Silver Stain: The most sensitive non-radioactive colorimetric stain, capable of detecting as little as $0.1\text{--}1\text{ ng}$ of protein per band. It relies on the binding of silver ions ($\text{Ag}^+$) to protein functional groups followed by chemical reduction to metallic silver ($\text{Ag}^0$).
• Coomassie Brilliant Blue (R-250 / G-250): Standard sensitivity, detecting approximately $10\text{--}50\text{ ng}$ of protein.
• Amido Black: Lower sensitivity, detecting approximately $50\text{--}100\text{ ng}$ of protein.

Correct option: (4) Silver stain