Which of the following is true for all parallelograms?

For all parallelograms, it is true that opposite sides are parallel and congruent. This fundamental property is what defines a parallelogram. In a parallelogram, the two pairs of opposite sides maintain both parallelism and equal length. This parallelism results from the definition of a parallelogram itself, ensuring that each pair of opposite sides is congruent to one another.

Other choices do not hold true for all parallelograms. For example, while certain types of parallelograms, such as rectangles, may have equal sides or right angles, this is not a characteristic of all parallelograms. Additionally, the diagonals of most parallelograms are not necessarily equal in length; this holds true only for specific cases such as rectangles or squares. Therefore, the statement regarding opposite sides being parallel and congruent is the only universally applicable property among all parallelograms.