The correct answer is: A. The police will be informed by them.
The sentence “They will inform the police” is in the future tense. The subject of the sentence is “they” and the verb is “will inform.” The object of the verb is “the police.” In order to form the passive voice, we need to move the object of the verb to the front of the sentence and make it the subject. We also need to change the verb to the past participle form. In this case, the past participle form of “inform” is “informed.” The sentence in the passive voice is: “The police will be informed by them.”
The other options are incorrect because they do not follow the rules of English grammar. Option B, “The police will inform them,” is in the active voice. Option C, “The police are informed by them,” is in the passive voice, but it does not use the correct verb tense. Option D, “Informed will be the police by them,” is not a grammatically correct sentence.