A uniform electric field is present in the region between infinite parallel plane plates A and B and a uniform electric field is present in the region between infinite parallel plane plates B and C. When the plates are vertical, is directed to the right and to the left. The signs of the charges on plates A, B and C may be

the signs on the plates are      A +    B-    C +

Explanation:

The electric field for an infinite plate is perpendicular to it, this field is outgoing if the charge on the plate is positive and incoming if the charge on the plate is negative.

Let's analyze the situation presented, we have two infinite plates A and B with the elective field directed to the right, therefore the charge of the plate must be

            plate A    positive charge

           plate B     Negative charge

We also have a third plate C and they indicate that the field between B and C is directed to the left, therefore

           plate B     negative charged

            plate C   positive charged

in short the signs on the plates are

    A +    B-    C +