Final answer:
The Roman Empire fell due to its vast territory becoming too large to manage effectively, combined with internal political strife and increasing external threats from better-organized barbarians and the Sassanids of Persia, which overwhelmed its stretched military and financial resources.
Step-by-step explanation:
The fall of the Roman Empire can be attributed to a combination of factors that include internal weaknesses and external pressures. Statement A, which asserts that the territory under Roman rule became too large to govern effectively, provides the best explanation for why the Roman Empire came to an end.
The empire's vastness made it increasingly difficult to manage and protect, especially with the deteriorating quality of Roman leadership and the challenges of internal political strife. This, coupled with growing threats from outside, including more populous and better-organized barbarian groups and the aggressive Sassanids of Persia, overwhelmed the Roman military and political structures.
The military was stretched thin across an expansive frontier, and as conflicts became a regular occurrence, the strain on resources and the military's ability to respond effectively to threats increased. Additionally, the loss of large territories due to frequent-barbarian invasions reduced tax revenues significantly, further weakening Roman power.