Ans. is 'c' i.e., -55 mv * The action potential, which is recorded using an intracellular electrode, has following phases : -1) Resting stage:# This is the resting membrane potential before the action potential. At rest, membrane is said to be "polarized" because - 70 mV negative membrane potential (RMP) that is present. Strictly speaking, it is not a part of the action potential.2) Depolarization phase:# The potential shoots up to + 35 mV in less than a millisecond. It is due to opening of "voltage gated" Na+ channels which causes influx of sodium, so inside negative charge becomes positive.# From -70 mV to -55 mV (firing Ivel or threshold point) the voltage gated sodium channles open partially. This portion of AP (-70 mV to -55 mV) is called "prepotential" or "foot ofAP".# Once the threshold level (-55 mV) is reached voltage gated Na+ channels open completely and the potential shoots up suddenly. It is called depolarization, since it represents loss of the original polarity of cell membrane.3) Repolarization phase:# The potential drops to near resting level, i.e., reversal to original polarity (repolarization). At the start of repolarization (end of depolarization), Voltage - gated Na+ channels close and Voltage - gated K+ channels open which cause efflux of K+ out of the cell. So, inside positive charge once again becomes negative due to efflux of positive charge (K+).4) After depolarization:# The repolarization is very fast till 70% of repolarization has been acheived. After that it is slows down. This slower phase of repolarization is called 'after depolarization5) After-hyperpolarization:# During repolarization phase K+ channels open and there is K+ efflux. The membrane voltage falls back (repolarizes) to resting potential, i.e., -70 mV. At this voltage, K+ channels close, but quite slowly.# Therefore, more K+ ions leak out, causing a brief period in which the voltage falls below -70 mV, i.e. hyperplorization. Then, K+/Na+ pump works to re-establish resting potential by pumping Na+ ions back out and K+ ions back into the cells.