**Core Concept:** Resting membrane potential is a crucial parameter in understanding neuronal excitability. It is maintained by the selective permeability of various ions through membrane channels and transporters. The primary ions involved are sodium (Na+), potassium (K+), calcium (Ca2+), and chloride (Cl-).

**Why the Correct Answer is Right:** Resting membrane potential is mainly determined by the selective permeability of potassium ions (K+). Potassium ions have a large negative charge, which drives them to move out of the cell through potassium channels and pumps. This results in a higher concentration of potassium ions outside the cell, creating a negative charge on the cell membrane.

**Why Each Wrong Option is Incorrect:**

A. Sodium (Na+) is positively charged and would move out of the cell to lower its concentration, not contributing to the resting membrane potential.

B. Calcium (Ca2+) is involved in various cellular processes but does not directly determine resting membrane potential due to its low charge and concentration difference.

C. Chloride (Cl-) is negatively charged but has a smaller concentration difference compared to potassium, therefore less contributing to the resting membrane potential.

**Clinical Pearl:** Understanding resting membrane potential is essential for understanding neuronal excitability and the role of ion channels in various neurological conditions like epilepsy, where ion channel abnormalities can lead to abnormal neuronal excitability and seizures.

**Correct Answer:** D. Potassium (K+) is the primary ion responsible for determining the resting membrane potential.