**Core Concept:**
The correct answer pertains to the ion channels and their role in determining the resting membrane potential (RMP) within neurons and other excitable cells. Resting membrane potential is the voltage difference between the inner and outer cellular compartments in a cell, primarily determined by the selective permeability of ion channels present in the cell membrane. The cell membrane, also known as the plasma membrane, is selectively permeable to certain ions, which in turn influences the RMP.

**Why the Correct Answer is Right:**
The correct answer, **D**, refers to the permeability of potassium ions (K+). Potassium ions are crucial for maintaining the RMP in neurons and other excitable cells. Potassium ions are carried out of the cell by voltage-gated potassium channels, which open when the membrane potential becomes more negative (hyperpolarizes), thereby reducing the net negative charge inside the cell. This helps to stabilize the RMP and prevent the cell from depolarizing too rapidly upon stimulation.

**Why Each Wrong Option is Incorrect:**

**A:** Sodium (Na+) permeability is not the primary determinant of RMP. Although sodium ions (Na+) contribute to the membrane potential, their permeability is not as significant as potassium (K+) ions in maintaining the RMP.

**B:** Calcium (Ca2+) permeability is not as crucial in determining RMP as potassium (K+) or sodium (Na+) permeability. Calcium ions play a role in cellular signaling, but their permeability does not directly influence the RMP.

**C:** Chloride (Cl-) permeability does not directly impact the RMP as significantly as potassium (K+) or sodium (Na+) ions. While chloride ions (Cl-) can influence the membrane potential, their permeability has less impact on RMP compared to potassium (K+) and sodium (Na+) ions.

**Clinical Pearl:**
Understanding the role of different ions in determining membrane potential is essential for understanding neuronal excitability and electrical signaling in the body. The correct answer, potassium (K+), is crucial for maintaining the RMP, ensuring that cells do not depolarize too rapidly upon stimulation. This concept is essential for understanding the electrical behavior of excitable cells, such as neurons, and how they generate and transmit signals.

**Correct Answer:** Potassium (K+) permeability