Final answer:
The neurotransmitter is released from the structure at b via exocytosis, where vesicles merge with the cell membrane to expel their contents, facilitating signal transmission across neurons.
Step-by-step explanation:
The structure at b releases neurotransmitter through the process of exocytosis. This mechanism involves vesicles containing neurotransmitters that fuse with the plasma membrane, allowing the contents to be released to the exterior of the cell. This process is essential in the transmission of signals across the synaptic cleft between neurons. During this process, the neurotransmitter is eventually diffused across the cleft and binds to receptor proteins on the postsynaptic membrane, continuing the signal transmission.