Upon adding hydrochloric acid in copper oxide, it turns to blue-green color which is due to the formation of

The correct answer is B, Copper (II) chloride. Black copper(II) oxide reacts with dilute hydrochloric acid to form copper(II) chloride and water, and the solution turns blue-green because of the copper(II) ions. The equation is CuO + 2HCl → CuCl2 + H2O. This is the textbook example, taught in Class 10, that metal oxides are basic in nature: they react with acids to give a salt and water, just as bases do in neutralisation. The colour comes from copper in its +2 state; copper(II) compounds such as copper sulphate are typically blue or green. Option A is wrong because 'copper hydrochloride' is not a real compound name. Option C is wrong because copper(I) chloride is a white solid in which copper is in the +1 state, and it does not form a blue-green solution here. Option D is wrong because a hydride contains hydrogen bonded to a metal, and no hydride forms when an oxide meets an acid. Exam tip: metal oxide + acid → salt + water; CuO is black, CuCl2 solution is blue-green.