**Core Concept**
The resting membrane potential (RMP) of a neuron is primarily determined by the movement of ions across the cell membrane, with potassium (K+) and sodium (Na+) being the key players. The RMP is a result of the balance between the outward flow of K+ ions and the inward flow of Na+ ions.

**Why the Correct Answer is Right**
The correct answer is K+ because it is the ion that contributes maximally to the development of RMP. This is due to the high permeability of the cell membrane to K+ ions, which allows them to flow out of the cell and establish a negative charge inside the cell. The high concentration of K+ ions inside the cell and their low concentration outside the cell create a large concentration gradient, driving the outward flow of K+ ions. This outward flow of K+ ions is balanced by the inward flow of Na+ ions, but the net effect is a negative RMP.

**Why Each Wrong Option is Incorrect**
**Option A:** This option is not provided, so it cannot be explained.
**Option B:** While sodium (Na+) ions do play a role in the development of RMP, they do not contribute maximally to it. The inward flow of Na+ ions is balanced by the outward flow of K+ ions, but the concentration gradient for Na+ ions is not as large as that for K+ ions.
**Option C:** This option is not provided, so it cannot be explained.

**Clinical Pearl / High-Yield Fact**
The RMP is a critical factor in the functioning of neurons, and changes in the RMP can lead to a range of neurological disorders. For example, a depolarized RMP can lead to hyperexcitability and seizures, while a hyperpolarized RMP can lead to decreased excitability and paralysis.

**Correct Answer:** K+ Potassium ions.