Welcome to the much-misunderstood present subjunctive. :) The answer is "confess". "Confesses" is indicative, meaning that it's an actual event. However in this case, we have a demand by the policeman, so the action is a conceived idea and not a real event.

confess is the right answer, that's subjunctive !

I confess you confess he/she confesses we confess they confess you confess The policeman is demanding that the thief confesses the crime.

I confess fdmaxey is right. :))))))

confess "Confesses" would be used more in the conditional. "If he confesses, we can avoid a full trial."