Which of the following dyes is an example of an azo dye?

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Congo red is an example of an azo dye. It contains two azo ( - N = N - ) groups and is synthesized by coupling tetrazotised benzidine with two molecules of naphthionic acid. Indigo is a vat dye. Martius yellow is a nitro dye. Malachite green is a triphenylmethane dye. Answer: Congo red