**Core Concept**
Exocytosis is a cellular process where vesicles fuse with the plasma membrane, releasing their contents to the outside environment. This process is crucial for neurotransmitter release, hormone secretion, and other cellular functions. The correct ion required for exocytosis is essential for triggering the fusion of vesicles with the plasma membrane.

**Why the Correct Answer is Right**
Calcium ions (Ca2+) play a pivotal role in exocytosis by binding to specific receptors on the vesicle membrane, triggering a conformational change that facilitates the fusion of vesicles with the plasma membrane. This process is mediated by the SNARE complex, a protein complex that brings the vesicle and plasma membrane together, allowing the contents of the vesicle to be released. The influx of calcium ions is often triggered by an action potential or other signaling events, making calcium a crucial ion for exocytosis.

**Why Each Wrong Option is Incorrect**
**Option A:** Potassium ions (K+) are essential for maintaining the resting membrane potential and repolarizing the membrane after an action potential but are not directly involved in exocytosis.
**Option B:** Sodium ions (Na+) are important for the initiation of the action potential but do not play a direct role in the exocytosis process.
**Option D:** Magnesium ions (Mg2+) are involved in many cellular processes, including muscle contraction and neuronal function, but are not directly required for exocytosis.

**Clinical Pearl / High-Yield Fact**
It's essential to remember that calcium channels are often targeted by medications used to treat conditions such as hypertension and angina. For example, calcium channel blockers can be used to relax blood vessels and improve blood flow.

**✓ Correct Answer: C. Calcium**