all four angles are right angles -wrong otherwise it would be a rectangle
the diagonals bisect each other- correct
all four sides are congruent - wrong that would be a Rhombus
the diagonals are congruent - wrong, this is the case in a rectangle, but in a parallelogram, because of the angle one side has to be shorter than the other
so b. the diagonals bisect each other is always true and therefore the solution