Before the Fall of the Byzantine Empire,how Were the Ottomans

Before the fall of the Byzantine Empire,how were the Ottomans able to organize into a power?

A) They benefited by little Mongol presence in Anatolia.
B) Osman emerged as an effective leader who conquered different area in Anatolia
C) From a base in the Dardanelles, they were able to conquer Greece and the Balkans.
D) They claimed to be loyal to the Byzantine Emperor.
E) All of these.