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The fusion of vesicles containing neurotransmitters with the membrane of the synaptic knob is primarily triggered by an influx of calcium ions. When an action potential reaches the synaptic knob, it causes depolarization of the neuron's membrane. This depolarization opens voltage-gated calcium channels, allowing calcium ions to flow into the cell.

The increase in intracellular calcium concentration serves as a signaling mechanism that prompts the vesicles to move toward the synaptic membrane and fuse with it. This fusion process is essential for the release of neurotransmitters into the synaptic cleft, where they can bind to receptors on the postsynaptic neuron and propagate the signal.

In summary, calcium ions play a crucial role in the process of synaptic transmission by facilitating the release of neurotransmitters, making calcium influx the correct choice in this context. Other ions such as sodium, magnesium, and chloride have various functions in neuronal activity and excitability, but they do not directly mediate the fusion of neurotransmitter vesicles in this specific process.