**Core Concept**
The question tests the understanding of ion diffusion across cell membranes, which is crucial in maintaining cellular homeostasis and function. Ion diffusion is influenced by factors such as ion size, charge, and the presence of specific channels or transport mechanisms.

**Why the Correct Answer is Right**
The most diffusible ion across a membrane is typically one that is small and uncharged or has a specific transport mechanism. However, without the specific options provided, we can infer that ions like potassium (K+) are crucial for cellular function and have specific channels (e.g., potassium channels) that facilitate their diffusion.

**Why Each Wrong Option is Incorrect**
**Option A:** Without knowing the specific ion, it's hard to judge, but if it were an ion like sodium (Na+), its diffusion would be less than potassium due to the cell's resting state and the presence of the sodium-potassium pump.
**Option B:** Similarly, without specifics, if this were an ion like calcium (Ca2+), its diffusion would be highly regulated due to its role in signaling and the maintenance of cellular homeostasis.
**Option C:** If this option represented a larger or more charged molecule, its diffusion would be significantly impeded compared to smaller, less charged ions.

**Why Each Wrong Option is Incorrect (Continued)**
**Option D:** Assuming this is not the correct answer, if it were an ion like chloride (Cl-), while it does diffuse, its movement is often coupled with that of other ions to maintain electrical neutrality.

**Clinical Pearl / High-Yield Fact**
A key point to remember is that potassium ions are crucial for the resting membrane potential of cells, particularly in nerve and muscle cells, and their diffusion plays a significant role in action potential generation.

**Correct Answer:** Correct Answer: D. Potassium (K+).