What triggers the merging of synaptic vesicles with the presynaptic membrane? 1) the binding of neurotransmitters with their receptors 2) the influx of calcium from the arrival of an action potential in the axon end terminal 3) the destruction of neurotransmitters by enzymes in the synaptic cleft 4) the generation of a graded potential in the postsynaptic membrane

2) the influx of calcium from the arrival of an action potential in the axon end terminal

Explanation: During neurotransmission, action potential from the axon triggers the opening of the voltage gated calcium channels and there is influx of calcium ions into the presynaptic neuron. The influx of calcium ions in turn triggers the fusion of the synaptic vesicles with the synaptic membrane with the consequent release of the neurotransmitter into the synaptic cleft. The neurotransmitter released in to the synaptic cleft diffuses to the post synaptic membrane where it binds to its receptors to trigger cellular response.

i believe it is "they are not used up by reactions" as the answer to your question, text me back if this is correct because i just learned this.