Final answer:
When voltage-gated K+ channels of a resting neuron open, K+ leaves the neuron to repolarize the membrane.
Step-by-step explanation:
When voltage-gated K+ channels of a resting neuron open, K+ leaves the neuron. This process is called repolarization. As K+ ions leave the cell, the membrane potential becomes negative again, restoring the resting potential of the neuron. Therefore, the correct answer is B. K+ leaves the neuron.