Which of the following is a common ingredient in nail hardeners?

Calcium is commonly included in nail hardeners because it plays a significant role in strengthening the nails. It aids in the development of healthier nail structure by promoting mineralization and improving overall hardness. The presence of calcium contributes to reducing brittleness and enhances the nails' resilience against splitting and breaking.

In contrast, sodium, potassium, and magnesium do not provide the same level of benefits for nail strength. Sodium is more associated with fluid balance in the body rather than strengthening nails. Potassium's main roles include nerve function and muscle contraction, rather than cosmetic benefits for nails. Magnesium is important for various biological processes but does not specifically contribute to the hardening of the nails as effectively as calcium does. This makes calcium the most appropriate choice among the options given.