Ans. is 'c' i.e., Potassium Phases of Action Potentialo The action potential, which is recorded using an intracellular electrode, has following phases : -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.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 sodiumQ, 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-i- channels open completely and the potential shoots up suddenly. It is called depolarization, since it represents loss of the original polarity of cell membrane.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 andVoltage - gated K+ channels open which cause efflux of K+ out of the cellQ. So, inside positive charge once again becomes negative due to efflux of positive charge (K+).After depolarization : - The repolarization is very fast till 70% of repolarization has been acheived. After that it is slows down. This slowrer phase of repolarization is called 'after depolarization After-hyperpolarization : - The intracellular negativity overshoots the normal RMP of - 70 mV to reach a more negative value of about - 75 mV and then slowly retruns to the normal RMP of - 70 mV.