When the gravity is greater than the electric field force, when the ball moves from the lowest point to the highest point, the resultant force will do negative work and the speed will decrease. Then when the ball moves to the highest point A, the tension of the line will be minimum, and when it reaches the lowest point B, the speed and kinetic energy of the ball will be maximum.
When gravity is equal to the electric field force, the ball moves in a uniform circle, and the speed and tension of the filament remain unchanged.
When the gravity is less than the electric field force, the tension of the line must be the maximum when the ball moves to the highest point A, and the speed is the minimum when the ball reaches the lowest point. So a, b and c are all wrong.
D. When the ball moves from the lowest point to the highest point, the electric field force does positive work and the electric potential energy decreases. When the ball moves to the highest point A, the electric potential energy of the ball is the smallest and the mechanical energy is the largest. Therefore, d is correct.
So choose d