Scientists beleive that the universe is constantly expanding since the big-bang, and that the galaxies are moving away from the epicenter of said big-bang. Now this could mean A or B is the answers, however A. states that they move through empty space, which is true but doesn't explain how the universe is expanding. B does however explain this as space expanding, which accounts for all the empty space being created by the galaxies moving apart.
Hope this helps.
