B) question a different suspect.
The phrase "barking up the wrong tree" means that someone is going in the wrong direction with his thinking. It refers to a dog who has chased an animal up the tree and stays at the bottom bark at the animal which is supposedly in the tree the dog is barking at. However, the animal has jumped to a different tree (or went up a different one in the first place) so the dog is "barking up the wrong tree" since the animal is not actually in the tree the dog is barking up into. The detectives should question a different subject and not just ask different questions because the suspect says that he'd been busy during the crime, so he feels that he is completely innocent and the police are looking at the wrong person just like the dog barked up the wrong tree.