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 there is a short, one or more coils will have a significantly lower voltage drop compared to the others. This method is effective at locating the specific coil(s) with the short. The other options are not as effective for locating a short in the shunt field. Isolating each coil and using a megohmmeter (B) can identify a ground fault but may not pinpoint the location of a short. Visual inspection of the commutator (C) would not directly reveal a shunt field short. Using a growler and hacksaw blade (D) is more appropriate for testing armature windings, not the shunt field.