Final answer:
An action potential is a large change in electrical charge from negative to positive inside a neuron, caused by the influx of sodium ions and the outflow of potassium ions.
Step-by-step explanation:
An action potential can be described as (d) A large change in electrical charge from negative to positive inside the cell. This process occurs when a neuron receives a stimulus strong enough to cause a sudden influx of positive sodium ions (Na+) into the neuron, which reduces the membrane potential and causes depolarization. The inside of the neuron becomes positively charged relative to the outside. Following this, potassium ions (K+) flow out of the cell, which repolarizes the membrane, restoring its negative charge.
The action potential is an all-or-nothing event that travels down the axon as an electric current. Depolarization and repolarization occur due to the opening and closing of voltage-gated ion channels and are necessary for the transmission of signals along neurons.