Which of the following indicators is commonly used to visualize bands of amino acids?

A) ninhydrin
B) bromocresol green
C) potassium permanganate
D) vanillin dip

Ninhydrin is commonly used to visualize bands of amino acids in various techniques such as paper chromatography and thin layer chromatography. When ninhydrin reacts with the primary amine group in the amino acid, it produces a purple-blue color, making it easy to detect and quantify the amino acid bands in the chromatogram. Bromocresol green and potassium permanganate are not used for visualization of amino acids, while vanillin dip is used for detection of carbohydrates.