Step 1: Identify the meaning of the sentence.
The sentence suggests that the army has received some form of communication or instruction. Therefore, the verb should imply receiving a warning or alert.
Step 2: Evaluate the options.
% Option
(1) Awakened: Incorrect. This doesn't fit the context of being prepared to control floods.
% Option
(2) has been raining: Incorrect. This option is unrelated to the context of readiness or preparation.
% Option
(3) Warned: Correct. The army is being alerted or warned to be ready.
% Option
(4) Wanted: Incorrect. This does not fit the context of preparedness.