Which of the following examples best illustrates a structural role of a mineral rather than an enzymatic cofactor or electrolyte role?

Sodium and potassium creating an electrochemical gradient across cell membranes that permits action potentials in nerve and muscle tissues.

Calcium forming hydroxyapatite crystals that provide bone rigidity.

Iron binding oxygen reversibly in hemoglobin and transporting it through the bloodstream as part of heme proteins important for oxygen delivery to tissues.

Magnesium stabilizing ATP for kinase activity across many metabolic pathways, acting as a necessary cofactor in energy transfer reactions during glycolysis and other enzymatic cascades.