**Core Concept**
The resting membrane potential (RMP) is the difference in electrical charge between the inside and outside of a cell when it is at rest. It is primarily determined by the concentration gradients of ions, particularly potassium, across the cell membrane.

**Why the Correct Answer is Right**
The RMP is mainly influenced by the movement of potassium ions (K+) out of the cell through the potassium leak channels. This movement creates a negative charge inside the cell due to the loss of positively charged potassium ions. The cell membrane is selectively permeable to potassium ions, allowing them to move freely out of the cell, which results in a net negative charge inside the cell. The Nernst equation, which calculates the equilibrium potential for a given ion, shows that the equilibrium potential for potassium is more negative than that for sodium, making potassium the primary determinant of the RMP.

**Why Each Wrong Option is Incorrect**
**Option A:** Sodium ions (Na+) play a role in generating the action potential, but their concentration gradient does not primarily determine the resting membrane potential.
**Option C:** Chloride ions (Cl-) can also contribute to the resting membrane potential, but their effect is less pronounced than that of potassium ions.
**Option D:** Calcium ions (Ca2+) have a significant role in various cellular processes, but they are not primarily responsible for determining the resting membrane potential.

**Clinical Pearl / High-Yield Fact**
The resting membrane potential is a critical determinant of neuronal excitability, and changes in the RMP can lead to various neurological disorders, such as epilepsy and neuropathic pain.

**✓ Correct Answer: B. Potassium**