How do myelin sheaths increase the speed with which action potentials are propagated along an axon?

The myelin sheath are the glial cells which present in the neurons. It acts as an insulator for the impulse. Inside the myelin sheath the impulse propagates in high speed. Because there is no depolarization occured in it. Therefore, the nerve impulse jumps from one sheath to other and it is called saltatory movement.  

The gap between the two myelin sheath is called nodes of ranvier. It contains sodium ions and have voltage gated channels. When the action potential passes through the myelin sheath, the depolarisation happens due to entry of excess sodium ions into the non - myelinated sheath / nodes of ranvier.  

When there is excess sodium ions present in the axon, again repolarization occurs and action potential occurs in the myelin sheath. Myelin sheath contains lipids which enhance the action potential.

c is the correct answer hope this !

for a different answer set the answer would be because the oxygen in the water had depleted (apex)