Answer:
Correct answer is C. The assassination of Julius Cesar .
Step-by-step explanation:
Option C is not a reason as Caesar was killed in 44 BC, while the Western Roman Empire fell in 476 AD. There is a gap of over 500 years.
A was a reason as Germanic tribes were constantly pressuring the Empire, which eventually collapsed under their pressure.
B and D are also reasons, as frequent plagues crippled the empire and after Constantine The Great the emperors were incompetent to preserve the strength of the Empire.