Are frequent intense aches of the head or neck that are not caused by another physical disorder.

Migraines are frequent intense aches of the head of neck that are not caused by another physical disorder. Severe pain may usually be felt on either one side or both sides of the head. Pain usually is felt in the temples. Migraine could last for hours or couple of days. 

option a is correct i.e.

explanation:

in context to the membrane of nerve impulse, the outer membrane of neuron is semi permeable that is only sodium ions can pass through the membrane i.e. smaller ions can pass through the membrane while larger ions cannot pass through it.