The correct answer is A) sounding each bar with a lightweight hammer. The most effective method of locating a loose commutator bar in a DC motor is to sound each bar with a lightweight hammer. This technique allows the operator to detect any looseness or cracking in the commutator bars by the sound produced when tapped. This is the recommended procedure outlined in marine engineering regulations and guidelines for maintaining and troubleshooting DC motors. The other options are less effective. Visual inspection (B) may not detect subtle looseness, a torque wrench (C) is not the appropriate tool, and jiggling by hand (D) is an imprecise method that can miss issues.