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Calcium pumps and ion channels are associated with the usage of action potential to release a neurotransmitter which is held within numerous synaptic vesicles.
When a calcium pump opens, the vesicles release neurotransmitters and begin to fuse with the plasma membrane. They then diffuse into the synaptic cleft and travel across to the cell, which causes the sodium, or ion, channels to open allowing the neurotransmitters through.
While the synaptic bulb is located on an axon, it resides on the axon terminal, the end. When thinking about this question remember that a synapse is what occurs between neural cells, and potentially other cells, so it would naturally be an area at the end of, or between, them.
However, if you are allowed two answers, some dendrites may be on the receiving end of the neurotransmitters and would thus have to use their sodium, or ion, channels. Technically A & D are correct, but your edited question of just calcium pumps would be synaptic bulbs.
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