Final answer:
Depolarization is the process most closely associated with an end-plate potential at the neuromuscular junction, where acetylcholine induces an influx of Na+ ions, potentially leading to an action potential and muscle contraction.
Step-by-step explanation:
The word or phrase most closely associated with an end-plate potential at the neuromuscular junction is b. Depolarization.
At the neuromuscular junction, the neurotransmitter acetylcholine (ACh) is released from the nerve terminal. When ACh binds to its receptors on the motor end plate, it causes an influx of Na+ ions through the ligand-gated channels. This influx of Na+ ions leads to a local depolarization of the sarcolemma, resulting in an end-plate potential.
The increase in Na+ ion concentration inside the muscle fiber reduces the membrane's resting potential, moving it closer to the threshold level required to initiate an action potential. If the end-plate potential is sufficiently large, it triggers an action potential that will be propagated along the muscle fiber, ultimately resulting in muscle contraction.