• Role of $Ca^{2+}$ in Neuromuscular Transmission: When a motor nerve action potential depolarizes the presynaptic motor nerve terminal, voltage-gated $P/Q$-type calcium channels open, allowing rapid influx of extracellular $\text{Ca}^{2+}$.
• Vesicular Fusion Mechanism: Intracellular $\text{Ca}^{2+}$ binds to synaptotagmin-1, triggering conformational changes in the core SNARE complex (syntaxin-1, SNAP-25, and synaptobrevin/VAMP). This forces the presynaptic synaptic vesicles to fuse with the presynaptic terminal membrane, releasing acetylcholine (ACh) into the synaptic cleft via exocytosis (Option 4).

Correct option: (4) Fuse the presynaptic vesicle with the presynaptic membrane, thus releasing the transmitter