Concept:The Biuret test detects proteins by forming a violet complex in an alkaline medium.Explanation:Protein is first treated with sodium hydroxide solution.This provides the alkaline condition needed for the test.Then a dilute solution of copper sulphate is added to the mixture.Copper ions react with peptide bonds of the protein.A positive result is indicated by a violet or purple colour.Thus, copper sulphate is the chemical required.Answer:B. copper sulphate