Concept:The phrase "read us the riot act" is an idiom that means giving a severe warning to stop misbehaving.
Explanation:"Read us the riot act" does not refer to an actual act or to riots.
It is used when someone in authority strongly tells others to stop causing trouble.
Option A is wrong because it gives a literal meaning.
Option B is wrong because it mentions "not to be involved in riots," which is too specific.
Option D is opposite in meaning.
So the most appropriate meaning is a stern warning to stop making trouble.
Answer:C. warned us to stop making trouble.