**Core Concept**
The equilibrium potential for an ion is a critical concept in understanding how ions move across cell membranes. It is the electrical potential at which the concentration gradient of an ion is exactly balanced by its electrical gradient, resulting in no net movement of the ion across the membrane.

**Why the Correct Answer is Right**
The equilibrium potential (E) for an ion is calculated using the Nernst equation, which takes into account the ion's charge (z), the temperature (T), the gas constant (R), and the concentrations of the ion inside (C_i) and outside (C_o) the cell. The Nernst equation is given by:

E = (RT/zF) * ln(C_i/C_o)

where F is the Faraday constant. This equation shows that the equilibrium potential is determined by the ratio of the ion concentrations inside and outside the cell, as well as the ion's charge.

**Why Each Wrong Option is Incorrect**
**Option A:** This option is incomplete and does not provide a valid equation for calculating the equilibrium potential.
**Option B:** This option is incorrect because it does not take into account the ion's charge (z) and the ratio of concentrations (C_i/C_o).
**Option C:** This option is incorrect because it is a simplified version of the Nernst equation and does not account for the temperature (T) and the gas constant (R).

**Clinical Pearl / High-Yield Fact**
The Nernst equation is a fundamental concept in understanding how ions move across cell membranes, and it has important implications for understanding various physiological and pathological processes, such as nerve conduction and muscle contraction.

**Correct Answer:** C. E = (RT/zF) * ln(C_i/C_o)