A rhombus is a quadrilateral whose sides are all of equal length. A square is a kind of rhombus in which the interior angles as well as the lengths of the sides are equal. One of the properties of quadrilaterals which have equal sides is that the diagonals, that is, the lines which link the opposite corners always bisect each other and are always perpendicular. The correct answer, therefore, is that they are always perpendicular.
