If in a quadrilateral ABCD, ∠D+∠B=180o, then it is not necessary that ABCD is a parallelogram.
As, opposite angles of parallelograms are congruent. Consecutive angles are supplementary. If one angle is right, then all angles are right. The diagonals of a parallelogram bisect each other.
So, ∠D+∠B=180o is not necessary to be ABCD is a parallelogram.