The correct answer is: A. open armature winding
A brush is a sliding electrical contact that transfers current from a stationary part of an electrical circuit to a rotating part. In a universal motor, the brushes are located on the commutator, which is a cylindrical metal ring with a series of copper segments. The commutator is connected to the armature winding, which is the rotating part of the motor.
When the motor is turned on, the brushes make contact with the commutator and current flows from the brushes to the armature winding. This current creates a magnetic field that causes the armature to rotate.
If the armature winding is open, there will be no current flow and the motor will not rotate. This can cause the brushes to spark, as they will be rubbing against the commutator without any current flowing through them.
The other options are not as likely to cause brush sparking. A shorted armature winding will cause the motor to overheat, but it is not as likely to cause sparking. A shorted field winding will cause the motor to run slowly, but it is not as likely to cause sparking. High commutator mica can cause sparking, but it is not as common as an open armature winding.
In conclusion, the most common cause of brush sparking in a universal motor is an open armature winding.