Which of the following elements would produce coloured ions in aqueous solution?

Iron is the element that would produce colored ions in aqueous solution. This is because iron has variable oxidation states, which means that it can lose different numbers of electrons to form different ions. When iron ions are in solution, they can absorb specific wavelengths of light and reflect others, resulting in the appearance of color. The color of the ion depends on the oxidation state of iron and the ligands (molecules or ions that surround the metal ion) attached to it. For example, Fe2+ ions can form a complex with the ligand SCN- to give a blood-red color, while Fe3+ ions can form a complex with the ligand EDTA to give a blue color. In contrast, calcium, magnesium, and sodium ions do not usually produce colored ions in aqueous solution.