The diagram shows a charge moving into an electric field. the charge will most likely leave the electric field near which letter?  

Near Y.

In fact, 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. In this problem, we have a negative charge, which is deflected downward (in the opposite direction of the field lines), so it will leave the electric field near position Y.

Lexy is correct because angle r is between the incident wave,  n  = 0, and a diffracted wave.