Prolonged means longer than usual or extended. So, going based on this..
It can't be torn since that means something that is ripped to pieces or shreds. Something being longer than usual would have little to no (flipped) connection with that.
Meek means shy.. Why would something being long have an opposite meaning submissive?
Slack can mean many things, but for this let's just say it means that something's loose. Well, loose and long mean nothing similar, unless we're admitting that this has a lot of slack (no pun intended) and is hanging down pretty low, making it long. Unfortunately, it doesn't, so we'll have to scratch that out.
Now, brief is out perfect match; it means short, and what is the opposite of short? Long! So, we have our answer:
The closest antonym for the word prolonged is brief.
