Final answer:
The termination of acetylcholine's action is primarily achieved through its metabolism by acetylcholinesterase, which degrades ACh in the synaptic cleft, allowing the postsynaptic membrane to reset.
Step-by-step explanation:
The process most important in the termination of the action of acetylcholine is C) Metabolism by acetylcholinesterase.
Once acetylcholine (ACh) is released into the synaptic cleft, it binds to ACh receptors on the postsynaptic membrane to relay the nerve signal. To terminate its action and reset the postsynaptic membrane, acetylcholinesterase degrades acetylcholine in the synaptic cleft. Although ACh can also be reabsorbed by the presynaptic neuron or diffuse away from the synaptic cleft, the metabolism by acetylcholinesterase is the primary mechanism for ending its action. Inhibitors of acetylcholinesterase are used in medical treatments, such as for Alzheimer's disease, to increase neurotransmission by allowing acetylcholine to remain in the synaptic cleft longer, thereby enhancing its chance to bind to the postsynaptic receptors.