Final answer:
Factors that reduce the influx of Ca2+ into the synaptic end bulb, such as decreased extracellular [Ca2+] or the presence of a conotoxin that blocks voltage-gated Ca2+ channels, would most likely decrease neurotransmitter release at a nerve synapse.
Step-by-step explanation:
The question pertains to the factors that would most likely decrease neurotransmitter release at a typical nerve synapse. The release of neurotransmitters is initiated by an action potential reaching the axon terminals which opens voltage-gated Ca2+ channels, allowing Ca2+ to influx and facilitate the fusion of neurotransmitter vesicles with the presynaptic membrane. Thus, any factor that reduces the concentration of Ca2+ in the synaptic end bulb would likely decrease neurotransmitter release. Of the options given, both a decreased extracellular [Ca2+] concentration and a conotoxin that blocks voltage-gated Ca2+ channels would achieve this by preventing the necessary Ca2+ influx required for vesicle fusion.