Answer:
Muffled.
Step-by-step explanation:
The sentence reads -
"Troops quelled the unrest that occurred after the unpopular law went into effect."
The right word for the word "quelled" in accordance to the whole sentence will be "muffled". The word quelled means "to put an end to" "to stop" or "to end" which is in the same sense with "muffled" which also suggests something that is being "forced to be put out", "to stop it from happening or being out" or "to put a stop to, to prevent". So, the right meaning for the word "quelled" will be "muffled".