The correct answer is: C. brush axis is 90° electrical to field axis.
In a repulsion motor, the stator windings are arranged to produce a rotating magnetic field. The rotor windings are connected to two brushes that ride on slip rings. The brushes can be moved to align the rotor windings with the stator field in different ways. When the brushes are aligned with the stator field, there is no torque produced and the motor stops. When the brushes are at 90° electrical to the stator field, the maximum torque is produced.
Option A is incorrect because when the brush axis is 45° electrical to the field axis, there is a small amount of torque produced.
Option B is incorrect because when the brush axis coincides with the field axis, there is no torque produced.
Option D is incorrect because only option C is correct.