That car has been slowed.
Why is "down" required?
That car has already slowed. That car has already slowed down. That car already slowed down. etc. These should all work IMHO.
Delayed may be better
I cant see the word "has" in the option.
As an Indonesian, I'm clearly confuse. Why do you use "diperlambat" in here? There's no situations in which Indonesian said "diperlambat" in this context. Maybe try switching with "melaju dengan pelan" or something