The Sandmeyer reaction replaces the diazonium group of an aromatic diazonium salt with a halogen or a pseudohalogen using cuprous halide as a catalyst. To introduce chlorine, benzenediazonium chloride is treated with cuprous chloride dissolved in hydrochloric acid; the copper(I) species promotes a radical mechanism that displaces nitrogen gas and installs the chlorine on the aromatic ring, giving chlorobenzene. The option of dilute sulphuric acid and water is wrong because that simply hydrolyses the salt to phenol, not to a chloride. Hypophosphorous acid reduces the diazonium group, replacing it with hydrogen rather than chlorine, so it gives benzene. Aqueous sodium hydroxide would couple or decompose the salt and cannot deliver chlorine. The need for a copper(I) catalyst distinguishes the Sandmeyer route from the Gattermann variant, which uses copper powder and the corresponding acid to achieve the same overall result. This is a core NCERT diazonium transformation often tested in JEE Main, and it is valuable because chlorine cannot be placed where the amino group originally stood by direct ring chlorination. As a plausibility check, the loss of nitrogen gas and the clean replacement by chlorine confirm a one-for-one substitution of the diazonium group consistent with the Sandmeyer pathway.