Explanation

Paper bleaching relies on chlorine-based compounds such as chlorine dioxide, sodium hypochlorite and bleaching powder (calcium hypochlorite) to remove lignin and whiten pulp. Chlorobenzene is an organic solvent intermediate, ammonium chloride is a fertilizer and flux, and hydrochloric acid is a general laboratory and industrial acid — none is the standard paper-bleaching agent. The chlorine-based bleaching compound is therefore the correct choice. Hence option A is correct.