Depolarization is initiated when there is an influx of sodium inside the cell as opposed to repolarization, where potassium exits the cell (occurring after Na+ gates close). Remember that for every 3 Na+ in, there is 2 K+ out. This imbalance helps to stabilize the membrane.
I've looked us up ever since you posted it the only thing I can come up with is letter A because it talks a lot about Mountain chains that talks a lot about the ocean floor and the Hawaiian stuff so try a