A. Emperor Alexander Severus, who was assassinated by his troops after a defeat.
B. Roman Senade, which accepted some army generals as legitimate emperors.
C. Security of the border, which they were supposed to keep. While the were fighting among themselves, they neglected the defenses of the Emprire, resulting in invasions.
D. Crisis, which ended with the Reforms made by Diocletian.
The purpose of any program is to provide its listeners with good information regarding some topic which has relevance to the listener. Whether this is something political or something scientific totally depends on the programs we're talking about. It also depends on the format in which it will be told.
An independent regulatory agency is part of the executive branch. The answer to your question is C. I hope that this is the answer that you were looking for and it has helped you.
