Question

Which event led to the weakening of the Roman Empire?

a) growth of scientific thought in Rome

b) migration of religious fanatics to Rome

c) untimely death of Emperor Augustus

d) disruption of Roman trade by pirates

Answer 1

so called Antonian plague in 2nd century and crisis of 3d century event led to the weakening of the Roman Empire would be best answers for that 
i hope that helps

Answer 2

migration of religious fanatics to Rome Led to the weakening of the Roman empire.
The migration of religious fanatics turn many people in Rome to fight and oppose The killing of the barbarian. This make the barbarian able to roam free and conquered a lot of region in the Rome's land, which led to weakening of Roman empire.