Answer: b. axon hillock.
Step-by-step explanation:
Neurotransmitter are the chemical substance which acts as a signal for neurotransmission. At a region called the axon hillock the axon emerges from the soma. At axon hillock, nerve impulses are produced by the plasma membrane; the axon pushes these impulses away from the soma, or dendrites towards other neurons.
All the synaptic inputs are summed up at the axon hillock which further stimulates neurotransmitter release. Axon hillock is called as the trigger zone.
Hence, the correct option is b. axon hillock.