The correct answer is: A. are
The police are a group of people who are responsible for enforcing the law. They are typically armed and have the authority to arrest people who break the law. The police are on duty 24 hours a day, 7 days a week. They are always ready to respond to calls for help and to keep the public safe.
The other options are incorrect because they do not agree with the subject of the sentence, which is “the police.” The police are a plural noun, so the verb must also be plural. Option B, “is,” is a singular verb, so it is incorrect. Option C, “was,” is a past tense verb, so it is also incorrect. Option D, “have,” is a plural verb, but it is not the correct verb tense. The police are always on duty, so the verb must be in the present tense.
In conclusion, the correct answer to the question “The police . . . . . . . . on duty” is “A. are.”