Concept:Understanding phrasal verbs formed with "put" and choosing the one that matches the meaning of "issue/announce."
Explanation:A warning is something that must be made known to many people, like an official notice.
"Put out" is used when someone publishes, broadcasts, or issues a statement or warning.
For example: The authorities put out a public alert about the storm.
The sentence shows that the police made a warning known to all residents, so this meaning fits perfectly.
Other phrasal verbs like "put on," "put in," or "put off" do not carry the meaning of publishing a warning.
Thus, the blank should be filled with "put out."
Answer:D. put out