**Core Concept**
Resting membrane potential (RMP) is the difference in electrical charge between the inside and outside of a neuron at rest. It is primarily determined by the distribution of ions (sodium, potassium, and chloride) across the cell membrane and the selective permeability of the membrane to these ions.

**Why the Correct Answer is Right**
The RMP is mainly influenced by the potassium ions due to the high concentration of potassium ions inside the cell and the low concentration outside. The potassium leak channels, also known as potassium leak potassium channels, allow potassium ions to diffuse out of the cell, creating a negative charge inside the cell. This is primarily due to the high permeability of the cell membrane to potassium ions at rest. The sodium-potassium pump (Na+/K+-ATPase) helps to maintain this concentration gradient, but the RMP is primarily determined by the potassium ions.

**Why Each Wrong Option is Incorrect**
* **Option A:** This option is likely incorrect because the resting membrane potential is not primarily determined by the sodium ions. While sodium ions do play a role in the action potential, the RMP is more influenced by the potassium ions.
* **Option B:** This option is incorrect because the chloride ions do not play a significant role in determining the resting membrane potential. Chloride ions are more involved in the regulation of the membrane potential during the action potential.
* **Option D:** This option is incorrect because the sodium-potassium pump (Na+/K+-ATPase) helps to maintain the concentration gradient of potassium ions, but it is not the primary determinant of the RMP.

**Clinical Pearl / High-Yield Fact**
A key point to remember is that the resting membrane potential is a dynamic equilibrium that can be altered by changes in the concentration of ions, the permeability of the cell membrane, or the activity of ion pumps.

**Correct Answer: C. Potassium ions**