The correct answer is A) sounding each bar with a light weight hammer. This is the most effective method because it allows you to physically inspect the commutator bars and detect any loose connections by the sound they make when tapped. A loose commutator bar will have a dull, muffled sound compared to a properly secured bar. Visual inspection (B) may not always reveal a loose bar, and using a torque wrench (C) or jiggling by hand (D) are less reliable methods for detecting this type of issue. Proper maintenance and inspection of the commutator is crucial for the safe and efficient operation of a DC motor, so using the sounding technique (A) is the recommended approach per industry standards and best practices.