Please help! pararllel plates?

(C) is the one you're having trouble with. It seems to be asking about a charged particle between the plates. It seems to be asking the total amount of force on that particle, causing it to accelerate towards one of the plates and away from the other. One of the plates attracts the particle, because it has the opposite charge. The other plate repels it. You need to figure those two forces, and add them.