Final answer:
If sodium cannot cross the membrane of an axon due to a drug, the neuron is unable to generate an action potential as it prevents the depolarization phase from occurring.
Step-by-step explanation:
If a drug prevents sodium (Na+) from crossing the membrane of an axon, then d) the neuron is unable to generate an action potential. Action potentials are critical for neuronal communication and involve a temporary influx of sodium ions into the neuron, which depolarizes the membrane. The sequence of events that occur during an action potential include the initial depolarization where Na+ channels open and allow positively charged sodium ions to enter the cell, reversing the resting membrane potential. Following the depolarization, repolarization occurs as potassium (K+) channels open, allowing potassium ions to exit, thereby returning the membrane potential to a negative value. Finally, the neuron returns to its resting state, ready for the next action potential. If Na+ cannot enter the neuron, the initial depolarization cannot occur and thus, no action potential is generated.