In A.C. series motor compensating winding is employed to

Answer: Option C

In A.C. series motors, compensating winding is used to reduce sparking at the brushes.
When a series motor operates, the armature current and field winding current are in series. This leads to a higher chance of sparking at the brushes due to the commutation process. The compensating winding is placed in the pole faces of the motor and produces a magnetic field opposite to that produced by the armature current during commutation. This counteracts the armature reaction, reducing sparking at the brushes.
Therefore, option C is the correct choice.