**Core Concept**
In excitable tissues, such as neurons and muscle cells, the resting membrane potential is maintained by the selective permeability of the cell membrane to ions. At rest, the cell membrane is more permeable to potassium ions (K+) than to sodium ions (Na+).

**Why the Correct Answer is Right**
The correct answer is **C. Sodium ions (Na+)**. This is because the cell membrane at rest is relatively impermeable to sodium ions due to the presence of sodium-potassium pumps (Na+/K+ ATPase) and sodium channels that are inactivated at resting potential. The resting membrane potential is maintained by the passive diffusion of potassium ions out of the cell, while sodium ions are actively pumped out of the cell by the sodium-potassium pump. As a result, the cell membrane is less permeable to sodium ions than to potassium ions at rest.

**Why Each Wrong Option is Incorrect**
* **Option A:** Potassium ions (K+) are actually more permeable than sodium ions at rest, making this option incorrect.
* **Option B:** Chloride ions (Cl-) are more permeable than sodium ions at rest, but the question specifically asks about sodium ions, making this option incorrect.
* **Option D:** Calcium ions (Ca2+) play a crucial role in excitation-contraction coupling in muscle cells, but they are not the least permeable ion at rest, making this option incorrect.

**Clinical Pearl / High-Yield Fact**
In excitable tissues, the resting membrane potential is maintained by the balance between potassium ions diffusing out of the cell and sodium ions being actively pumped out of the cell. This balance is crucial for the generation and propagation of action potentials.

**Correct Answer:** C. Sodium ions (Na+)