Explanation

Demineralised water is produced by passing water first through a cation-exchange resin, which swaps dissolved cations such as Ca2+ and Mg2+ for H+ ions, and then through an anion-exchange resin, which swaps anions such as Cl- and SO4 2- for OH- ions. The released H+ and OH- combine to form pure water, leaving it free of soluble mineral salts. Microfiltration removes only suspended particles, Calgon's method merely sequesters hardness, and boiling only removes temporary hardness. Hence the resin-bed method is correct.