**Core Concept**
Potassium ions (K+) play a crucial role in maintaining the resting membrane potential and facilitating the generation of action potentials in excitable tissues, such as neurons and muscle cells. The selective permeability of the cell membrane to K+ ions allows for the efficient diffusion of these ions, contributing to the establishment of the resting membrane potential.

**Why the Correct Answer is Right**
At rest, the cell membrane is more permeable to K+ ions than to other ions, such as sodium (Na+). This is due to the presence of voltage-gated potassium channels, which allow K+ ions to flow out of the cell, resulting in a negative membrane potential. The concentration gradient of K+ ions, with a higher concentration inside the cell, also drives their diffusion out of the cell. This process helps to establish the resting membrane potential, which is a critical factor in the generation of action potentials.

**Why Each Wrong Option is Incorrect**
**Option A:** Sodium ions (Na+) are not the most diffusible ions in excitable tissue because the cell membrane is relatively impermeable to Na+ at rest, and the concentration gradient of Na+ ions favors their influx into the cell.

**Option C:** Phosphate ions (PO4-) are not diffusible ions in excitable tissue because they are not directly involved in the generation of action potentials. Phosphate ions play a crucial role in cellular metabolism, but they are not selectively permeable through the cell membrane.

**Option D:** Chloride ions (Cl-) are not the most diffusible ions in excitable tissue because the cell membrane is relatively impermeable to Cl- at rest. While Cl- ions can contribute to the resting membrane potential, they are not as selectively permeable as K+ ions.

**Clinical Pearl / High-Yield Fact**
The resting membrane potential is primarily determined by the diffusion of K+ ions out of the cell, establishing a negative membrane potential that is essential for the generation of action potentials. This concept is critical in understanding the physiology of excitable tissues and the pathophysiology of various neurological and muscular disorders.

✓ Correct Answer: B. K+