(a) Yes, the +7Q charge has a net force on it. The force from the +Q charge is directed up and to the right; the force from the -Q charge is directed down and to the right. The net force ends up pointing to the right.

(b) No, the -Q charge experiences a larger-magnitude net force. It has three forces acting on it, and all three are either directed left or they have a component directed left. The three forces acting on the +Q charge individually have the same magnitudes as the three forces acting on the -Q charge, but one is directed to the right while the other two have components going left...there is more cancellation for these forces than there is for the forces acting on the -Q charge.

(c) The two -2Q charges produce fields that cancel one another, so we can ignore those. The other two charges give identical fields, each one producing an electric field directed right with a magnitude of kQ/d². Therefore the net field is 2kQ/d² directed right.

(d) A>B>C. In all three cases the fields from the +Q and -Q do the same thing, so the difference between the three cases comes from the other charges in each case. In case C the fields from the -2Q charges cancel, which is why C ends up last in our list. In B we get partial cancellation of the fields from the +4Q and +3Q; in A we just get a big field from the +7Q charge. So, A has a larger field at the origin than B does.

(e) A. In each case we get a force on the -Q from the +Q. Then in C we get forces right and left, so there's some cancellation. In B we get a big net force which is mainly to the left, but again there is partial cancellation of the forces from the +4Q and the +3Q charges. In A there's really nothing to cancel the big force from the +7Q charge, so A wins.