**Core Concept**
Calcium plays a crucial role in various cellular processes, including muscle contraction, neurotransmitter release, and bone mineralization. The body tightly regulates calcium levels through a complex interplay of hormones and receptors. Understanding the active form of calcium is essential to appreciate its physiological effects.

**Why the Correct Answer is Right**
The active form of calcium is **calcium ions (Ca2+)**, which are generated by the release of calcium from intracellular stores or the influx of calcium from extracellular sources through voltage-gated calcium channels. This process is crucial for muscle contraction, as calcium ions bind to troponin and tropomyosin, allowing actin and myosin to slide past each other and produce muscle contraction. Additionally, calcium ions play a key role in neurotransmitter release and neuronal excitability.

**Why Each Wrong Option is Incorrect**
* **Option A:** Calcium phosphate is an inactive form of calcium, which is stored in bone and teeth. It does not participate in cellular processes directly.
* **Option B:** Calcium carbonate is a dietary supplement, not an active form of calcium. It is absorbed in the gut and then converted to calcium ions, which are then available for physiological processes.
* **Option D:** Calcium channel blockers are a class of medications that inhibit the influx of calcium ions into cells, leading to vasodilation and decreased cardiac contractility. They do not represent an active form of calcium.

**Clinical Pearl / High-Yield Fact**
Calcium ions (Ca2+) play a critical role in neurotransmitter release, and their dysregulation is implicated in various neurological disorders, including epilepsy and Parkinson's disease.

**Correct Answer:** C. Calcium ions (Ca2+).