Answer: D. "No one gave a thought to the older worlds of space as sources of human danger, or thought of them only to dismiss the idea of life upon them as impossible or improbable."
This quote is the one that better supports the theme because it states that humans were unprepared for the attack because they took their safety for granted. The world was to focused on itself to spare a thought on the "older worlds of space." Moreover, even when they did, they believed the idea of them having life to be impossible or improbable.