In Neurons, the Synaptotagmin-1 Protein Is Localized to Presynaptic Axon

In neurons, the synaptotagmin-1 protein is localized to presynaptic axon terminals. When activated, it facilitates the fusion of neurotransmitter-containing synaptic vesicles to the plasma membrane, leading to neurotransmitter release into the synaptic cleft. Which of the following would you expect to directly activate synaptotagmin-1 for this function?

A) Binding to Mg²⁺
B) Binding to Ca²⁺
C) Binding to phosphatidylcholine
D) Binding to K⁺
E) Binding to the K⁺ leak channels