Answer: C. Depolarization of the presynaptic membrane causes Ca²⁺ uptake into the neuron. Ca²⁺ acts as a signalling chemical inside the neuron and promotes neurotransmitter release. A: confuses K⁺ efflux, associated with repolarization, with the signal for neurotransmitter release. B: confuses Na⁺ influx, associated with depolarization, with the ion that signals release. D: reverses the direction: Ca²⁺ enters rather than leaves the presynaptic neuron. Remember: Ca²⁺ entry at the presynaptic membrane signals neurotransmitter release.