The correct answer is - A) a rising plume of magma from the mantle.
The Hawaiian Island chain is formed as a result of volcanic activity. It is not the typical volcanic activity seen around the world though, but it is a hot-spot volcanic activity. Unlike the other volcanoes that are located near the plate boundaries, the Hawaiian Islands are located in the middle of the Pacific Plate, with not a single plate boundary nearby.
There's an active hot-spot bellow the Pacific Plate, and it is so strong that it manages to break through its crust relatively easily as the Pacific Plate is moving above it. This rising plume of magma has led to the formation of the largest volcanoes in the world, and created a whole island chain. The process will continue, with the volcanoes that are active now stopping their activity in the future because the Pacific plate is dragging them away from the hot-spot. As they are dragged away though, the hot-spot will create new volcano, with new volcanic island rising above the ocean waters, increasing the number of islands in the Hawaiian Island chain.