Final answer:
Option A: Before an action potential can be generated, a neuron must experience a threshold stimulus that depolarizes the membrane to around -55 mV, which is the critical level needed to initiate an action potential.
Step-by-step explanation:
Before an action potential is generated, a threshold stimulus must occur. An action potential is an all-or-nothing event that begins when a neuron is stimulated by a sensory cell or another neuron, causing the target cell to depolarize toward the threshold potential. This threshold is a critical level of depolarization, generally about 15 millivolts (mV) more positive than the cell's resting membrane potential, commonly reaching around -55 mV. When this threshold is exceeded, voltage-gated Na+ channels open, leading to rapid depolarization of the neuron.
In the process of generating an action potential, as shown in the references provided, the first part of the neuron cannot fire another action potential during the refractory period, following an initial action potential. The specific channels involved, the voltage-gated Na+ channels, play a critical role in depolarization during the action potential. Thus, the correct answer to what must happen before an action potential is generated is c. Threshold stimulus.