In response A, the fact that b = c (as vectors) implies that the quadrilateral is a parallelogram and the fact that the dot product of a and c is zero implies that one of the angles is right, i.e., the quadrilateral is a rectangle (not necessarily a square). In response B, the quadrilateral is a parallelogram with a = c (equal as lengths, not as vectors). This is a rhombus, but not necessarily a rectangle because nothing is said about the angles.