Answer : It was on a major trade route between Europe and Asia. Constantinople, the Empire's capital, was one of the main hubs of the Silk Road (a major trade route spanning much of Asia), as well as being located on the Bosporus, the only sea route between the Black Sea and the Mediterranean.
Explanation:As for the other options:
1) It was not on an island, the empire mostly spanned Greece and Turkey. Its location was not unknown by its enemies either, the Byzantines were frequently attacked by Muslim emirates and kingdoms in modern-day Bulgaria and Serbia.
2) It was not surrounded by many friendly nations, the empire was at war with enemies on both sides of its frontier for much of its history - the main religion was Greek Orthodoxy, so neighbouring Catholic and Islamic states wanted to conquer it.
3) It did not control almost all the area of the former Roman Empire, it only controlled parts of the eastern section of the Roman Empire. The only exception to this was during the reign of the Emperor Justinian, who succeeded on conquering a large chunk of the western half. However, these territories were soon lost after his death.
