Which of the following compounds contains magnesium and chloride ions?

The compound that contains magnesium and chloride ions is magnesium dichloride, represented by the chemical formula MgCl2. In this formula, "Mg" stands for magnesium, and "Cl" stands for chlorine. The number "2" after chlorine indicates that there are two chloride ions for each magnesium ion in the compound.

In ionic compounds, the formula reflects the ratio of the ions to maintain electrical neutrality. Magnesium has a +2 charge, and each chloride has a -1 charge. Therefore, two chloride ions are needed to balance the +2 charge of the magnesium, resulting in the neutral compound MgCl2.

Other options do not contain both magnesium and chloride ions. MgSO4 contains magnesium and sulfate ions, CuCl2 contains copper and chloride ions, and CaCl2 contains calcium and chloride ions. Each of these compounds has a different metal or anion than those specified in the question. Therefore, only MgCl2 is the correct choice that includes both magnesium and chloride ions.