There is a common confusion whether 'police' is singular or plural. It really depends on the context of the sentence. When 'police' is a common noun, it's considered to be plural, since 'police' refers to the cops. When it is used as a proper noun, such as 'The London Police', it is a specific team. So, it's considered as a collective noun, and is hence, singular. Also, when it is specified that it is only one 'police officer', then it is singular.
In the given sentence, 'police' is used as a common noun, hence it is plural.
The verb must agree with the subject in terms of person and number. If the subject is singular, so is the verb. The same goes for a plural subject.
Option B: 'Want' is a plural verb. It agrees with the plural subject 'police'. Hence B is the correct option.
Option A: 'Wants' is a singular verb and does not agree with the subject here. Hence A is incorrect.
Options C and D: 'Is wanting' is grammatically incorrect.
'Wanting' is not used as a verb, it's used as an adjective (present participle that modifies nouns) or a gerund. Hence, it cannot form a verb phrase.
'Wanting' does not form the required tense. Hence these options are incorrect.