A. The police had been looking for the thief for months and finally caught him in the act.
Explanation:
The best answer choice is "The police had been looking for the thief for months and finally caught him in the act." Inconsistency or shift in a sentence voice confuses the reader. Shifts in the voice can make reading difficult and the meaning confusing. To correct the shift, both clauses in the sentence should be in the same (active) voice. This leaves the subject of both clause as "the police," which improves the flow of ideas.
