**Core Concept**
Smooth muscle contraction is initiated by depolarization of the plasma membrane, leading to calcium release from the sarcoplasmic reticulum (SR) and subsequent activation of contractile proteins. Unlike skeletal muscle, smooth muscle lacks T-tubules and relies on intracellular calcium signaling for contraction.

**Why the Correct Answer is Right**
In smooth muscle, depolarization of the plasma membrane (via ion channels) triggers opening of voltage-gated calcium channels (L-type Ca²⁺ channels) in the plasma membrane. This leads to an influx of extracellular Ca²⁺, which then binds to calmodulin. The Ca²⁺-calmodulin complex activates myosin light-chain kinase (MLCK), leading to phosphorylation of myosin and smooth muscle contraction. Thus, increased intracellular Ca²⁺ is the key event initiating contraction.

**Why Each Wrong Option is Incorrect**
Option B: Smooth muscle lacks T-tubules and does not undergo depolarization of T-tubules like skeletal muscle. The depolarization occurs only in the plasma membrane.
Option C: Inhibition of Na⁺ entry is not involved in initiating contraction; Na⁺ influx contributes to depolarization but not directly to contraction.
Option D: Acetylcholine binds to muscarinic receptors on the plasma membrane, not on the sarcoplasmic reticulum. It does not directly bind to SR receptors; instead, it triggers signaling pathways that lead to Ca²⁺ release.

**Clinical Pearl / High-Yield Fact**
In smooth muscle, contraction is primarily driven by extracellular Ca²⁺ influx through voltage-gated channels, not by SR release as in skeletal muscle. This makes Ca²⁺ signaling a key therapeutic target in conditions like hypertension and gastrointestinal motility disorders.

✓ Correct Answer: A. Cause opening of the calcium channel which leads to increase in Ca+2 contraction