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

• A. Sodium and potassium creating an electrochemical gradient across cell membranes that permits action potentials in nerve and muscle tissues.
• B. Calcium forming hydroxyapatite crystals that provide bone rigidity.
• C. Iron binding oxygen reversibly in hemoglobin and transporting it through the bloodstream as part of heme proteins important for oxygen delivery to tissues.
• D. 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.

Answer: (B)