Based on the standard diagram of an axon terminal and synapse (Figure 18.3 in NCERT Class 11), 'A' represents the Receptors present on the post-synaptic membrane that bind to released neurotransmitters. 'C' represents the Synaptic vesicles containing neurotransmitters within the pre-synaptic axon terminal. 'B' typically represents the Synaptic cleft (the gap), and neurotransmitters are the chemicals released.