**Core Concept**
The intracellular space, also known as the intracellular fluid (ICF), is the compartment within cells where various ions and molecules are present. The major cation in this space is crucial for maintaining cellular homeostasis, membrane potential, and various cellular functions.

**Why the Correct Answer is Right**
The correct answer is due to the significant presence of **potassium ions (K+)** within the intracellular space. Potassium ions play a vital role in maintaining the resting membrane potential, which is essential for proper neuronal and muscle function. The high concentration of potassium ions in the ICF is maintained through the action of the Na+/K+-ATPase pump, which actively transports sodium ions (Na+) out of the cell in exchange for potassium ions. This process helps to maintain a negative resting membrane potential, which is critical for the proper functioning of excitable cells.

**Why Each Wrong Option is Incorrect**
* **Option A:** Sodium ions (Na+) are the major cation in the extracellular space, not the intracellular space.
* **Option B:** Calcium ions (Ca2+) are present in both the extracellular and intracellular spaces, but they are not the major cation in the intracellular space.
* **Option D:** Magnesium ions (Mg2+) are also present in both spaces, but they are not the major cation in the intracellular space.

**Clinical Pearl / High-Yield Fact**
The ratio of sodium ions to potassium ions in the extracellular space versus the intracellular space is approximately 14:1, which is essential for maintaining proper cellular function and membrane potential.

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