Final answer:
The ions that flow through the postsynaptic ACh receptor are primarily Na⁺ and to a lesser extent, K⁺, resulting in depolarization of the membrane and excitation (a. Na⁺ influx, K⁺ efflux - Excitation).
Step-by-step explanation:
The ions that flow through the postsynaptic ACh (acetylcholine) receptor are primarily sodium ions (Na⁺) and to a lesser extent, potassium ions (K⁺). Upon the release of acetylcholine at the synapse, such as the neuromuscular junction, ACh binds to its receptor on the postsynaptic membrane. This binding opens ligand-gated ion channels which allows Na⁺ to influx into the cell and K⁺ to efflux but at a much slower rate. This moving of ions causes the membrane to depolarize, creating what is called an excitatory postsynaptic potential (EPSP). This increase in positivity inside the neuron makes it more likely that the neuron will fire an action potential.
The correct answer to the question is a. Na⁺ influx, K⁺ efflux - Excitation, as the influx of Na⁺ leads to excitation of the postsynaptic cell.