Which statement is always true for any parallelogram?

a. all four angles are right angles

b. the diagonals bisect each other

c. all four sides are congruent

d. the diagonals are congruent

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