**Core Concept**
Calcium ions play a crucial role in the regulation of skeletal muscle contraction and relaxation. The influx of calcium ions into the sarcoplasmic reticulum triggers the sliding filament theory, leading to muscle contraction. This process is essential for muscle function and is mediated by various molecular mechanisms.

**Why the Correct Answer is Right**
During muscle contraction, calcium ions bind to troponin and tropomyosin, causing a conformational change that exposes the myosin binding sites on the actin filaments. This binding event initiates the sliding filament mechanism, leading to muscle contraction. The sarcoplasmic reticulum, a specialized form of smooth endoplasmic reticulum, is responsible for regulating calcium ion concentrations within the muscle cell. The ryanodine receptor, a calcium channel, releases calcium ions from the sarcoplasmic reticulum, triggering muscle contraction.

**Why Each Wrong Option is Incorrect**
**Option A:** Incorrect because calcium ions in skeletal muscles primarily regulate contraction and relaxation, not neuromuscular transmission.
**Option B:** Incorrect because calcium ions do not primarily regulate muscle tone, although they do play a role in muscle relaxation.
**Option C:** Incorrect because calcium ions in skeletal muscles are not primarily involved in muscle repair or regeneration.

**Clinical Pearl / High-Yield Fact**
It is essential to note that disruptions in calcium ion regulation can lead to various muscle disorders, including malignant hyperthermia and hypocalcemia.

**Correct Answer: C.**