**Core Concept**
Excitation-contraction coupling in smooth muscles is a complex process that involves the transmission of the electrical signal to the contractile apparatus. This process is crucial for the regulation of smooth muscle contraction, which plays a vital role in various physiological functions such as blood pressure regulation, gut motility, and airway tone. The presence of intracellular calcium ions (Ca2+) is essential for initiating smooth muscle contraction.

**Why the Correct Answer is Right**
In smooth muscles, the electrical signal triggers the opening of voltage-gated calcium channels, allowing an influx of calcium ions into the cell. The increase in intracellular calcium concentration activates the contractile apparatus by binding to calmodulin, a calcium-binding protein. This complex then activates the enzyme myosin light chain kinase (MLCK), which phosphorylates the light chain of myosin, leading to muscle contraction. The presence of cellular calcium is therefore essential for initiating and maintaining smooth muscle contraction.

**Why Each Wrong Option is Incorrect**
**Option A:** The presence of troponin is essential for excitation-contraction coupling in skeletal and cardiac muscles, not in smooth muscles. Troponin is a complex of three proteins that regulates muscle contraction by binding to tropomyosin and blocking the myosin binding site on actin.
**Option B:** While high calcium concentrations can lead to sustained contraction, the presence of cellular calcium is essential for initiating contraction, not just sustaining it. The relationship between calcium concentration and contraction is complex, and sustained contraction is influenced by various factors, including the activity of calcium pumps and the sensitivity of the contractile apparatus.
**Option C:** Phosphorylation of actin is not required for contraction in smooth muscles. In fact, actin is not phosphorylated during muscle contraction; instead, the light chain of myosin is phosphorylated by MLCK.

**Clinical Pearl / High-Yield Fact**
Smooth muscle contraction is essential for various physiological functions, and disruptions in this process can lead to various diseases, including hypertension, asthma, and gastrointestinal disorders. Understanding the mechanisms of excitation-contraction coupling in smooth muscles is crucial for the development of targeted therapies for these conditions.

**✓ Correct Answer: D. Presence of cellular calcium is essential to cause muscle**