**Core Concept**
The coupling agent between electrical excitation and contraction of a muscle is a critical process that enables the transmission of electrical signals into mechanical force. This process involves a complex interplay between the sarcolemma, the T-tubules, and the sarcoplasmic reticulum.

**Why the Correct Answer is Right**
The correct answer is **Calcium ions (Ca²⁺)**. When an action potential reaches the muscle fiber, voltage-gated calcium channels in the T-tubules open, allowing an influx of Ca²⁺ ions into the sarcoplasm. This increase in intracellular Ca²⁺ concentration triggers the binding of Ca²⁺ to troponin and tropomyosin, leading to a conformational change in the actin-myosin complex and ultimately causing muscle contraction.

**Why Each Wrong Option is Incorrect**
* **Option A:** **Acetylcholine (ACh)** is a neurotransmitter that initiates the electrical excitation of the muscle fiber, but it is not the coupling agent between electrical excitation and contraction.
* **Option B:** **Potassium ions (K⁺)** are involved in the repolarization of the muscle fiber after contraction, but they are not directly involved in the coupling process.
* **Option C:** **Sodium ions (Na⁺)** are involved in the depolarization of the muscle fiber, but they are not the coupling agent between electrical excitation and contraction.

**Clinical Pearl / High-Yield Fact**
It's essential to remember that the coupling agent between electrical excitation and contraction is a calcium-dependent process, and disruptions in this process can lead to muscle disorders such as malignant hyperthermia or central core disease.

**Correct Answer:** C.