**Core Concept**
Excitable tissues, such as neurons and muscle cells, have a unique resting membrane potential due to varying permeabilities to different ions. The **resting membrane potential** is primarily maintained by the differential distribution of **sodium (Na+)** and **potassium (K+)** ions across the cell membrane.

**Why the Correct Answer is Right**
At rest, excitable cells are least permeable to **sodium ions (Na+)**. This limited permeability is crucial because the concentration gradient for Na+ is such that if the cell were very permeable to Na+, it would allow a rush of positively charged Na+ ions into the cell, depolarizing it and making it impossible for the cell to maintain a resting state. The **sodium-potassium pump** helps maintain this gradient, but the key factor is the low permeability to Na+.

**Why Each Wrong Option is Incorrect**
**Option A:** Potassium (K+) - Incorrect because at rest, excitable cells are relatively more permeable to K+ than to Na+, allowing K+ to leave the cell and contribute to the negative resting membrane potential.
**Option C:** Calcium (Ca2+) - Incorrect because while cells are also less permeable to Ca2+, the question context and the maintenance of resting potential make Na+ the more relevant ion in this scenario.

**Clinical Pearl / High-Yield Fact**
Remember, the resting membrane potential is crucial for proper neural and muscle function. Any disruption in ion permeability or pump function can lead to significant clinical issues, such as arrhythmias in the heart or neurological disorders.

**Correct Answer:** D. Sodium.