Ans.
The given diagram represent chemical synapse, which is defined as biological junction between two neurons and is responsible for the exchange of neural signals. The structure labeled with question mark is synaptic vesicle. The synaptic vesicles are small sac like structures that contain neurotransmitters. These neurotransmitters are released by these vesicles through expcytosis and bind to receptors present in another neuron.
Thus, the correct answer is option b). synaptic vesicle with neurotransmitter.'
