Final answer:
After long-term potentiation (LTP), the post-synaptic cell requires a higher concentration of neurotransmitter to respond.
Step-by-step explanation:
After long-term potentiation (LTP), the synapse's activity is expected to differ in the following way: the post-synaptic cell should require a higher concentration of neurotransmitter to respond. LTP is a process that strengthens the synapse's connection, making the post-synaptic cell more likely to respond to neurotransmitter release from the pre-synaptic cell. This means that a higher concentration of neurotransmitter would be needed to generate a response in the post-synaptic cell.
Thus, the correct answer is D: The post-synaptic cell should require a higher concentration of neurotransmitter to respond.