In this sentence we would be understanding the subject verb agreement. The rule states that some nouns are singular in form, but plural in meaning, take plural verb. "Police" is plural in meaning.
"was" is a singular verb in the past tense. It is also the wrong choice.
"are" is a plural verb used in present tenses. It is the correct option
"is" is a singular verb. It is the wrong choice.
"has been" is used with the present perfect tense, which is used when we want to talk about something that started in the past but is still going on right now in the present OR is still relevant / true today. It is also the wrong option.
Option C is correct.