**Core Concept**
The 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 movement of ions (charged particles) across the cell membrane, particularly the distribution of potassium (K+) and sodium (Na+) ions.

**Why the Correct Answer is Right**
The RMP is maximally affected by the movement of potassium ions (K+). The cell membrane is highly permeable to K+ ions, which tend to diffuse out of the cell down their concentration gradient. This movement of K+ ions out of the cell creates a negative charge inside the cell, resulting in a negative RMP. The Nernst equation, which calculates the equilibrium potential for a given ion, shows that the equilibrium potential for K+ is more negative than that for Na+, making K+ the primary determinant of the RMP.

**Why Each Wrong Option is Incorrect**
* **Option A:** Sodium ions (Na+) do play a role in the RMP, but their movement is not the primary determinant. The equilibrium potential for Na+ is more positive than the RMP, making it less influential.
* **Option B:** Chloride ions (Cl-) are also present in the RMP, but they have a relatively minor effect compared to K+ and Na+.
* **Option D:** Calcium ions (Ca2+) are not a significant contributor to the RMP, as their concentration gradient is not as steep as that of K+ and Na+.

**Clinical Pearl / High-Yield Fact**
The resting membrane potential is a critical factor in the generation of action potentials. Changes in the RMP, such as those caused by shifts in ion concentrations or changes in membrane permeability, can alter the excitability of neurons and contribute to various neurological disorders.

**Correct Answer: C. Potassium ions (K+)**