Final answer:
The membrane potential of the postsynaptic neuron moves closer to the threshold during an excitatory postsynaptic potential (EPSP) through a process called depolarization.
Step-by-step explanation:
When a postsynaptic potential (PSP), such as an excitatory postsynaptic potential (EPSP), occurs, the membrane potential of the postsynaptic neuron moves closer to the threshold that is necessary to trigger an action potential. This movement towards the threshold is known as depolarization because it involves a reduction in the difference in voltage between the inside and outside of the neuron.
Therefore, the correct answer is (b) depolarization. During this process, neurotransmitters bind to receptors on the postsynaptic neuron, causing ligand-gated ion channels to open, allowing positive ions to enter the cell. This influx of positive ions reduces the membrane's polarity, thus depolarizing it. If the membrane reaches the threshold of excitation, the neuron may fire an action potential.