If the police hadn't arrived to stop the gangs, there would have been a riot.
A.
If by the police the gangs hadn't been stopped, there would have been a riot.
B.
If the police hadn't stopped the gangs, there would have been a riot.
C.
If the gangs hadn't by the police been stopped, there would have been a riot.
D.
If the gangs hadn't been stopped, there would have been a riot.