A.C. series motor and D.C. series motor are both types of electric motors that are used for various applications. However, there are certain differences between the two types of motors. Let's discuss each option to understand why the correct answer is option 'E' - all of the above.

- Smaller brush width:
In a D.C. series motor, the brushes are typically wider compared to an A.C. series motor. The wider brush width helps to distribute the current more evenly and reduce the risk of arcing. On the other hand, the smaller brush width in an A.C. series motor allows for better commutation and reduces the chances of sparking and brush wear.

- Less number of field turns:
The field turns in a motor refer to the number of turns of wire in the field winding. In a D.C. series motor, the field turns are generally higher compared to an A.C. series motor. This is because D.C. series motors require a higher magnetic field strength to generate the required torque and speed. In contrast, A.C. series motors operate at higher frequencies and can achieve the required magnetic field strength with fewer field turns.

- More number of armature turns:
The armature turns in a motor refer to the number of turns of wire in the armature winding. In an A.C. series motor, the armature turns are typically higher compared to a D.C. series motor. This is because A.C. series motors operate at higher frequencies and require more armature turns to generate the necessary torque and speed. The higher number of armature turns also helps in reducing the armature resistance and increasing the motor's efficiency.

- Less air gap:
The air gap in a motor refers to the distance between the rotor and the stator. In an A.C. series motor, the air gap is generally smaller compared to a D.C. series motor. This is because A.C. series motors operate at higher frequencies, and a smaller air gap helps to reduce the magnetic reluctance and improve the motor's performance.

In conclusion, all of the given options are correct. A.C. series motors have a smaller brush width, less number of field turns, more number of armature turns, and a smaller air gap compared to D.C. series motors. These differences arise due to the different operating principles and requirements of the two types of motors.