**Core Concept**
The equilibrium potential is a measure of the electrical potential difference required for a specific ion to be at equilibrium across a cell membrane, with no net movement of the ion. It is calculated using the Nernst equation, which takes into account the ion's concentration gradient and the permeability of the membrane.

**Why the Correct Answer is Right**
The equilibrium potential for a particular ion is determined by its concentration gradient and the membrane's permeability to that ion. In the case of the highest equilibrium potential, it is typically for potassium ions (K+), as the concentration gradient for potassium is usually the steepest across the cell membrane. This is due to the high permeability of the membrane to potassium ions, which allows them to easily diffuse down their concentration gradient.

**Why Each Wrong Option is Incorrect**
**Option A:** This option is incorrect as it does not specify the ion for which the equilibrium potential is being calculated.
**Option B:** This option is incorrect as it is not a common ion for which the equilibrium potential is calculated.
**Option C:** This option is incorrect as it is not the ion with the highest equilibrium potential.

**Clinical Pearl / High-Yield Fact**
The Nernst equation is used to calculate the equilibrium potential for a specific ion, and it is an important concept in understanding how ions move across cell membranes. The equilibrium potential for potassium ions is typically around -90 mV, which is the highest among all ions.

**Correct Answer:** C. Potassium.