The correct answer is: A. Just ahead of magnetic neutral axis.
The magnetic neutral axis is the point at which the magnetic field of the stator and rotor are equal and opposite. When the brushes are rocked ahead of this point, the current in the commutator segments is interrupted at a point where the magnetic field is weak, which minimizes sparking.
Option B is incorrect because the brushes should be rocked ahead of the magnetic neutral axis, not in it. Option C is incorrect because the brushes should be rocked ahead of the magnetic neutral axis, not behind it. Option D is incorrect because the brushes must be rocked ahead of the magnetic neutral axis in order to achieve sparkless commutation.