What was Byzantium's major advantage?

First things first. The Byzantines Never had a big army, not even in the Justinian era of the great conquests. So it is definitely not A. I trust D is an "offspring" of E, so it's down to 3 choices. i guess the truth rests between C and E. Also a great factor that you don't mention and is maybe even of greater importance is religion. For 1000 years, over a Millennium, the Byzantines drew their strengh from the union of so many different cultures and tribes because they had all the same faith, in the One, Holy,Catholic and Apostolic Church, which is now known as the greek Orthodox Church.I hope is helped you