Final answer:
C) Potassium ions
Phase 3 of the fast response action potential is characterized by an increase in conductance to potassium ions (C), which exit the cell through voltage-gated potassium channels to repolarize the membrane.
Step-by-step explanation:
Phase 3 of the fast response action potential is due, in part, to an increase in conductance to potassium ions (C). During this phase of the action potential, voltage-gated potassium channels open, allowing potassium ions to exit the cell.
This outflux of potassium ions is instrumental in repolarizing the membrane, which means it helps in returning the cell to its resting membrane potential after the action potential has peaked. The increased potassium conductance ensures the action potential does not last indefinitely and that the cell is ready for a subsequent action potential if necessary.
During the action potential, sodium ions initially flow into the cell which triggers depolarization. This initial influx of sodium is then followed by the outflow of potassium ions during phase 3, crucial for repolarization. This sequential opening and closing of ion channels and the flux of ions are central to the generation and termination of an action potential.