To solve this problem, we need to identify where the receptor sites for neurotransmitters are located. Let's analyze each option:$\newline \bullet$Option 1: Pre-synaptic membrane.$\newline$ - The pre-synaptic membrane is the part of the neuron that releases neurotransmitters into the synaptic cleft.$\newline$ - It contains synaptic vesicles filled with neurotransmitters but does not have receptor sites for neurotransmitters.$\newline \bullet$Option 2: Tips of axons.$\newline$ - The tips of axons, also known as axon terminals, are involved in the release of neurotransmitters.$\newline$ - They do not contain receptor sites for neurotransmitters.$\newline \bullet$Option 3: Post-synaptic membrane.$\newline$ - The post-synaptic membrane is the part of the neuron or muscle cell that receives the neurotransmitter signal.$\newline$ - It contains receptor sites that bind to neurotransmitters, allowing the signal to be transmitted to the next cell.$\newline$ - Therefore, receptor sites for neurotransmitters are present on the post-synaptic membrane.$\newline \bullet$Option 4: Membranes of synaptic vesicles.$\newline$ - Synaptic vesicles store neurotransmitters within the pre-synaptic neuron.$\newline$ - Their membranes do not have receptor sites for neurotransmitters.$\newline$Based on this analysis, the correct answer is Option 3: Post-synaptic membrane.