The correct answer is A) applying AC voltage to the shunt field circuit and measuring the voltage drop across each field coil. This is the correct approach because it allows you to identify a short in the shunt field by measuring the voltage drop across each field coil. If one coil has a significantly lower voltage drop, it indicates a short in that coil. This method is effective in locating the short without having to dismantle the motor. The other options are not as suitable for this task. B) using a growler and hacksaw blade is more appropriate for checking the armature for shorts, not the shunt field. C) isolating each coil and using a megohmmeter can work, but it requires more time-consuming disassembly. D) visual inspection of the commutator would not reliably detect a short in the shunt field.