Answer: C. arrival of invaders.
Outside pressures were especially problematic for the Byzantine Empire, in particular, the arrival of invaders. The most important of these were the Ottoman Turks. By 1373, Emperor John V was forced (as were the Serbian princes and Bulgarian rulers) to become a vassal of the Turks due to the failing economy of the empire and their inability to defend themselves. The rise of Murad II as a sultan proved even more challenging to Byzantium, as he laid siege to the city. Finally, his successor, Mehmed II successfully conquered the city in 1453.