I can't really choose any of the options, as the question is far more complicated than that. The Western world is somewhat of an appendage to the old Greek and Roman Empires. The West hasn't had a one all powerful emperor to tie it all together, but Christianity became the unifying force in Europe...until the Reformation anyway.
Then you had the Enlightenment and the exploration of different political theories. The Industrial Revolution. Then the most important for wealth, which was colonization and empire.
It can't be understated how militaristic the West was and to some extent still is either. Strong militaries, innovation, political expression, Christianity, etc are all important factors.
Also, when it comes to the "West", it isn't so much when "it" will decline, but what nation will carry the torch of Western civilization. It was the Greeks, the Romans, the British, the French, the Spanish, etc..all at one time or another.