Okay, let's tackle this question. The question is about determining the magnitude of the electrical difference across a membrane. Hmm, so I need to recall the basic principles of membrane potential.

First, I remember that the resting membrane potential is primarily due to the concentration gradients of ions like K+ and Na+, and the permeability of the membrane to these ions. The Nernst equation comes to mind, which calculates the equilibrium potential for a specific ion. But wait, the question is about the overall membrane potential, not just one ion. Oh right, the Goldman-Hodgkin-Katz equation is used when multiple ions are involved, taking into account their concentrations and permeabilities.

So the correct answer would be the Goldman equation. Let me check the options. The user didn't list them, but common distractors might include the Nernst equation, sodium-potassium pump, or maybe the action potential. The Nernst equation is for a single ion, so if an option had that, it's incorrect. The sodium-potassium pump contributes to the potential but isn't the primary determinant of the membrane potential's magnitude. The action potential is a change in membrane potential, not the resting one.

Clinical pearl: The Goldman equation considers K+, Na+, and Cl- concentrations and their permeabilities. It's crucial in understanding resting membrane potential. Students often confuse Nernst and Goldman equations, so that's a key point.

**Core Concept**
The question assesses understanding of membrane potential calculations. The **Goldman-Hodgkin-Katz (GHK) equation** determines the resting membrane potential by considering ion concentrations and membrane permeability, while the **Nernst equation** calculates equilibrium potential for a single ion.

**Why the Correct Answer is Right**
The **Goldman equation** integrates the relative contributions of K⁺, Na⁺, and Cl⁻ ions, weighted by their membrane permeability. It accounts for both concentration gradients and differential permeability, making it the most accurate model for resting membrane potential. For example, high K⁺ permeability dominates in resting neurons, resulting in a membrane potential near K⁺’s Nernst potential (~-90 mV).

**Why Each Wrong Option is Incorrect**
**Option A:** *Nernst equation* applies to single ions (e.g., K⁺ equilibrium potential) but ignores other ions and permeability ratios.
**Option B:** *Sodium-potassium pump* generates gradients but does not directly determine membrane potential.
**Option C:** *Action potential* is a transient change, not the baseline electrical difference.

**Clinical Pearl / High-Yield Fact**
Membrane potential depends on **permeability ratios**, not just ion concentrations. For instance, even if extracellular K⁺ rises (e.g., hyperkalemia), the membrane potential depolarizes because K⁺ permeability dominates.

**Correct Answer: C. Goldman equation**

✓ Correct Answer: C. Nernst equation