The correct answer is B. At its height, the empire comprised nearly 1.7 million square miles of land and water.
The most important difficult to defend the Roman empire against invasion were the extension of its territory and its frontier.
Other aspects were positives because a great population allowed to have a bigger army, the paved roads helped to the faster communication and sending of troops, and the extension of the Mediterranean Sea functioned as a natural barrier, even more if we consider that the Roman navy was much more superior than their enemies.
The Barbarian Germanic tribes made a big pressure around the Northern and West Northern Limes (Frontier in Latin Language), so they had to send a great number of soldiers to protect the Empire against them.
Finally, when the empire was in a week situation, it was destroyed by the invasion of those people.