The correct answer is: A. thin wire placed at the top of the slots.
The main winding of a split phase motor is made of thin wire because it needs to be able to carry a large current. The current in the main winding creates a magnetic field that rotates the motor. The magnetic field is strongest at the top of the slots, so the thin wire is placed there to maximize the strength of the magnetic field.
The other options are incorrect because they do not describe the correct placement of the wire in the slots. Option B would result in a weaker magnetic field, option C would result in a thicker wire that would be more difficult to wind, and option D would result in a magnetic field that is not strongest at the top of the slots.