Final answer:
The description that does NOT guarantee that a quadrilateral is a parallelogram is: a quadrilateral with two opposite sides parallel.
Step-by-step explanation:
A quadrilateral is a parallelogram if it satisfies certain conditions. Let's go through each description to determine which one does not guarantee that a quadrilateral is a parallelogram:
A. A quadrilateral with both pairs of opposite sides congruent - This condition is true for parallelograms, so it guarantees that a quadrilateral is a parallelogram.
B. A quadrilateral with the diagonals bisecting each other - This condition is true for parallelograms, so it guarantees that a quadrilateral is a parallelogram.
C. A quadrilateral with consecutive angles supplementary - This condition is true for parallelograms, so it guarantees that a quadrilateral is a parallelogram.
D. A quadrilateral with two opposite sides parallel - This condition is required for a quadrilateral to be a parallelogram, but it is not sufficient. It is possible for a quadrilateral to have two opposite sides parallel and still not be a parallelogram (e.g., a trapezoid).
Therefore, the correct answer is option D: a quadrilateral with two opposite sides parallel.