What triggers the release of acetylcholine from a synaptic terminal?

calcium ions

Explanation:

The neurotransmitters are the compounds which help in the transmission of electric signals through the neurons.  The neurotransmitters are stored in the vesicles present in the axon terminal or the synaptic cleft like acetylcholine in motor neurons.

The release of the acetylcholine from the synaptic vesicles depends on the presence of the calcium ions in the external environment that is when the neurons bring the signal, the calcium voltage channels gets opened. The influx of the calcium takes place which binds to the receptor and allows the fusion of the vesicle to the membrane and the release of the neurotransmitter.

Thus, calcium ion is the correct answer.

1. a tropical dry forest carries out more rapid nutrient cycling