**Core Concept**
The movement of ions across cell membranes is a critical process in maintaining cellular homeostasis and regulating various physiological functions. The most diffusible ion is typically the one with the smallest size and charge, allowing it to easily pass through the lipid bilayer of the cell membrane.

**Why the Correct Answer is Right**
The most diffusible ion across the membrane is **Potassium (K+)**. This is because potassium ions are small in size and have a relatively small charge, making it easier for them to diffuse through the lipid bilayer of the cell membrane. Additionally, the cell membrane is more permeable to potassium ions due to the presence of potassium leak channels, which facilitate the movement of potassium ions out of the cell.

**Why Each Wrong Option is Incorrect**
**Option A:** Sodium (Na+) is not the most diffusible ion across the membrane due to its larger size and charge compared to potassium ions.
**Option B:** Calcium (Ca++) is not the most diffusible ion across the membrane due to its large size and multiple positive charges, making it less permeable to the cell membrane.
**Option C:** Chloride (Cl-) is not the most diffusible ion across the membrane due to its larger size compared to potassium ions and its tendency to be trapped within the cell due to the negative charge.

**Clinical Pearl / High-Yield Fact**
The movement of potassium ions across the cell membrane plays a crucial role in maintaining the resting membrane potential and regulating various physiological functions, including muscle contraction and nerve impulse transmission.

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