**Core Concept:** In excitable tissues, ions play a crucial role in generating and propagating electrical impulses. The most diffusable ions are those that can easily move across cell membranes.

**Why the Correct Answer is Right:** In excitable tissues such as neurons and heart cells, rapid and efficient diffusion of ions is essential for the generation and conduction of electrical signals. The membrane potential is maintained by the selective permeability of ion channels and transporters. The most diffusable ions are those with the smallest size, lowest charge, and weak interactions with the lipid bilayer. In excitable cells, these ions are:

1. **Option A (Sodium, Na+):** Sodium ions are small and uncharged, allowing them to diffuse rapidly across the cell membrane. Their over- or under-activation leads to depolarization or hyperpolarization, respectively, which is crucial for action potential initiation and propagation.
2. **Option B (Potassium, K+):** Potassium ions are also small and uncharged, promoting rapid diffusion across the membrane. Their central role in repolarization of the action potential is essential for maintaining membrane potential homeostasis and preventing runaway depolarization.
3. **Option C (Chloride, Cl-):** Although chloride ions are also small and uncharged, their relatively larger size and strong interaction with the lipid bilayer reduces their diffusion rate compared to sodium and potassium ions.
4. **Option D (Calcium, Ca2+):** Calcium ions are large, charged, and have strong interactions with the membrane lipids, making them less diffusable than sodium, potassium, and chloride ions.

**Why Each Wrong Option is Incorrect:**

- **Option C (Chloride):** Chloride ions are larger in size and have stronger interactions with the lipid bilayer, resulting in slower diffusion rates compared to sodium and potassium ions. This makes them less suitable for rapidly propagating electrical signals.
- **Option D (Calcium):** Calcium ions are larger, highly charged, and have strong interactions with the lipid bilayer. This reduces their diffusion rate in excitable tissues, making them unsuitable for rapid propagation of electrical signals.

**Clinical Pearls:** Understanding the role of these ions in maintaining membrane potential and generating action potentials is essential for understanding physiological processes in the nervous and cardiovascular systems. The correct ions (sodium, potassium, and chloride) contribute to the generation and propagation of action potentials, ensuring efficient neuronal and cardiac communication.