The correct option is B Na+ ions from extracellular fluid to intracellular fluid
In the resting nerve fibre, the cytoplasm inside the axon has a high concentration of K ions and a low concentration of Na ions, in contrast to the fluid outside the axon. Therefore, the inside of the nerve fibre carries a negative charge as compared to the positive charge on the outside.
During the propagation of a nerve impulse, action poteintial results in a movement of Na ions from the extracellular fluid to the inside of the nerve fibres.
Thus a positive charge develops inside the nerve fibre.