**Core Concept**
The maximum equilibrium potential, also known as the equilibrium potential, is the electric potential difference across a cell membrane that would cause no net flow of an ion if the ion channels were open. This concept is crucial in understanding the resting membrane potential and the behavior of ions during action potentials.

**Why the Correct Answer is Right**
The maximum equilibrium potential is highest for potassium ions (K+) because the equilibrium potential is determined by the Nernst equation, which takes into account the concentration gradient and the charge of the ion. For K+, the equilibrium potential is calculated as E_K = (RT/F) \* ln([K+]\_outside / [K+]\_inside), where RT/F is the thermal voltage, and [K+]\_outside and [K+]\_inside are the concentrations of K+ outside and inside the cell, respectively. Since the concentration of K+ is typically higher outside the cell than inside, the equilibrium potential for K+ is more positive, resulting in a maximum equilibrium potential of approximately +90 millivolts.

**Why Each Wrong Option is Incorrect**
**Option A:** Sodium (Na+) has a much lower equilibrium potential compared to K+ because its concentration is higher inside the cell, resulting in a more negative equilibrium potential.
**Option C:** Chloride (Cl-) has an equilibrium potential close to the resting membrane potential, which is around -70 millivolts, and is not the ion with the maximum equilibrium potential.

**Clinical Pearl / High-Yield Fact**
The Nernst equation is a fundamental concept in understanding the behavior of ions during action potentials. Remember that the equilibrium potential is determined by the concentration gradient and the charge of the ion, which makes K+ the ion with the maximum equilibrium potential.

**Correct Answer:**
✓ Correct Answer: B. K+