**Core Concept**
The relaxation of smooth muscle is a complex process that involves a decrease in intracellular calcium ion concentration. This decrease is mediated by various mechanisms, including the sequestration of calcium ions into the sarcoplasmic reticulum and the extracellular space. The reduction in free intracellular calcium ions triggers a series of downstream effects that ultimately lead to muscle relaxation.

**Why the Correct Answer is Right**
The reduction in free intracellular calcium ions triggers the dephosphorylation of myosin light chains, which in turn leads to the detachment of myosin heads from actin filaments. This detachment reduces the force of muscle contraction, resulting in relaxation. Additionally, the decrease in intracellular calcium ions also inhibits the phosphorylation of myosin light chains, further contributing to muscle relaxation.

**Why Each Wrong Option is Incorrect**
**Option A:** This option is incorrect because it does not accurately describe the effect of reduced intracellular calcium ions on smooth muscle relaxation. The correct answer is not related to the contraction of skeletal muscle.
**Option B:** This option is incorrect because it is not directly related to the relaxation of smooth muscle. Although calcium channels do play a role in muscle contraction, their role is not directly related to the reduction of intracellular calcium ions.
**Option C:** This option is incorrect because it describes a mechanism of muscle contraction, not relaxation. The increased phosphorylation of myosin light chains leads to muscle contraction, not relaxation.

**Clinical Pearl / High-Yield Fact**
A key point to remember is that the relaxation of smooth muscle is a complex process that involves multiple mechanisms, including the reduction of intracellular calcium ions. Understanding these mechanisms is crucial for treating conditions such as hypertension, asthma, and erectile dysfunction.

**Correct Answer:** D.