The ball has a resultant force on it, after the string is cut, equal to mg downwards, its weight, and Eq horizontally, the force due to the field E on its charge q.
If, as the question says, the electric field is uniform and horizontal, then the force Eq doesn't change in magnitude or direction as the ball moves.
Similarly, the force mg doesn't change.
So the ball moves with uniform acceleration acted upon by a constant force equal to the vector sum of mg and Eq.
The parabola would be relevant if the electric field was vertical and not horizontal as it states here.
Or if there was no electric field and the ball was given an initial horizontal velocity only.
Assuming you copied it correctly, the word 'constant' is missing from the mark scheme where it says 'the ball experiences a (constant) vertical and horizontal force'.