Answer:
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 +
