Introduction:
Armature reaction is a phenomenon that occurs in DC motors when the armature current generates a magnetic field that interacts with the main magnetic field produced by the field winding. This interaction can cause flux distortion, which affects the performance of the motor. The question asks which type of DC motor experiences more pronounced flux distortion due to armature reaction.

Explanation:
The correct answer is option 'A', which states that the flux distortion caused by cross magnetizing armature reaction is more pronounced in a DC shunt motor. Let's discuss why this is the case:

1. Armature Reaction:
Armature reaction occurs when the armature current flows through the armature coils, creating a magnetic field that interacts with the main magnetic field produced by the field winding. This interaction causes the flux distribution in the air gap to deviate from the ideal uniform distribution.

2. Cross Magnetizing Armature Reaction:
Cross magnetizing armature reaction refers to the armature flux that is perpendicular to the main field flux. This type of armature reaction occurs in all types of DC motors, but its effect is more pronounced in some motors than others.

3. DC Shunt Motor:
In a DC shunt motor, the field winding is connected in parallel with the armature winding. This arrangement allows the field winding to have a relatively high resistance compared to the armature winding. As a result, the field current is mostly independent of the armature current.

4. Effect of Cross Magnetizing Armature Reaction:
In a DC shunt motor, the cross magnetizing armature reaction causes the armature flux to shift in the direction perpendicular to the main field flux. This shift reduces the effective flux available for producing torque, resulting in a decrease in motor performance.

5. Pronounced Flux Distortion:
The flux distortion caused by cross magnetizing armature reaction is more pronounced in a DC shunt motor due to its specific design characteristics. The independent nature of the field winding allows it to maintain a relatively constant flux level, which makes the armature reaction more significant compared to other types of DC motors.

Conclusion:
In conclusion, the flux distortion caused by cross magnetizing armature reaction is more pronounced in a DC shunt motor due to its design characteristics. The independent field winding in a DC shunt motor allows the cross magnetizing armature reaction to have a greater impact on the motor's performance, resulting in more pronounced flux distortion.