Yellow stains on the fingers while working with nitric acid are due to the presence of

When concentrated HNO₃ is added to proteins, a white precipitate is formed which on boiling turns to yellow. Upon cooling, when we add NaOH to it, the color changes to orange indicating the presence of proteins. A yellow stain is often observed on the skin when it comes in contact with nitric acid. The reason for the yellow stain is the xanthoproteic reaction which is a test for proteins.

Hence, the correct answer is option A) proteins.