**Core Concept**
Free or ionized calcium is the biologically active fraction of calcium that is not bound to proteins or other molecules in the blood. It plays a crucial role in various physiological processes, including muscle contraction, nerve transmission, and blood coagulation.

**Why the Correct Answer is Right**
The percentage of free or ionized calcium in the body is approximately 45%. This is because the majority of calcium in the blood is bound to proteins such as albumin, which maintains an equilibrium between free and bound calcium. The free or ionized calcium is the fraction that is not bound to proteins and is available for physiological processes.

**Why Each Wrong Option is Incorrect**
**Option A:** This option is incorrect because the percentage of free or ionized calcium is not significantly affected by the binding of calcium to albumin.
**Option B:** This option is incorrect because the percentage of free or ionized calcium is not directly related to the total calcium concentration in the blood.
**Option C:** This option is incorrect because the percentage of free or ionized calcium is not influenced by the presence of other ions such as magnesium.

**Clinical Pearl / High-Yield Fact**
It's essential to note that the percentage of free or ionized calcium can be affected by changes in albumin levels, which can occur in conditions such as liver disease or severe malnutrition. Therefore, total calcium levels should be interpreted in conjunction with albumin levels to accurately assess the free or ionized calcium.

**Correct Answer: C. 45%**