Final answer:
Hyperpolarization occurs when the membrane potential becomes more negative than the resting potential, in this case moving from -70mV to -90mV. The correct answer is option c).
Step-by-step explanation:
When the membrane potential changes from -70mV to -90mV, this is known as hyperpolarization. During the action potential, the first channels to open are sodium ion channels, leading to depolarization as sodium ions enter the cell, increasing the positive charge up to about +30mV.
Following this, the potassium channels open, allowing potassium to leave the cell and bring the charge back towards the negative, a process known as repolarization. However, the membrane potential doesn't just return to the resting value of -70mV.
It overshoots this due to the delayed closing of potassium channels, momentarily causing the membrane potential to become more negative than the resting potential, and thus causing hyperpolarization.