you haven't attached the diagram, but i assume that this diagram is what you were talking about
Answer:
near Y
Explanation:
the electric field lines goes from a positive charge to a negative charge. This means that a positive charge would move in the same direction of the field lines, while a negative charge would move in the opposite direction of the field lines. the field lines are created from +vely charged plate to -vely charged plate so the negative charged particles moves towards the lower plate which is positively charged, and opposite to the direction of field lines.