Answer:
The upper plate is positive charged.
Step-by-step explanation:
From the information we have, we know that the plastic bead, carrying a negative charge of -6 nC, is suspended between the two plates by the force of the electric field between them.
If it is suspended, this means that it is at rest. As we have two forces acting on the bead (gravity and electrostatic force), and gravity is always downward, this means that the electrostatic force must be upwards.
If this force is due to the field, as the force on a negative charge is opposite to the direction of the field, this means that the electric field created by the charge on the plates must be pointed downward.
If it points downward, as the electric field has the same direction that it would have a positive test charge, this means that the charge would go down also, away from the positive charged plate, which must be the upper one.