Explanation:
The charge (positive or negative) is always on the surface in the conductors. So, if the plates have substantial thickness, the charge is distributed throughout the surface. We know from the question that the bottom plate is charged positively, so an equal amount of negative charge must be distributed on the lower surface of the upper plate.
The direction of the E-field is always directed from positive to the negative charge. Since the plates have flat surfaces, the electric field lines are straight, except the edges. In most college level questions this effect is negligible, that is why in most questions the plates are considered very large.
