What must be true for a quadrilateral to be classified as a parallelogram?

For a quadrilateral to be classified as a parallelogram, it is essential that at least two pairs of opposite sides are parallel. This characteristic is fundamental to the definition of a parallelogram and ensures that the opposite sides have equal slopes, creating parallel lines.

In a parallelogram, the properties that emerge from this definition include that opposite sides are equal in length, and opposite angles are equal. Additionally, consecutive angles are supplementary. Therefore, the parallel nature of the sides is crucial, as it establishes the overall structure and properties of the shape.

The other options present conditions that either do not necessarily define a parallelogram or are too restrictive. For example, while all sides being equal does characterize a special type of parallelogram known as a rhombus, it is not a requirement for all parallelograms. Similarly, having at least one right angle describes a rectangle or square, specific types of parallelograms but not parallelograms in general. Only one pair of sides being equal does not ensure that the quadrilateral maintains the properties required for classification as a parallelogram. Thus, the requirement of having at least two pairs of opposite sides that are parallel is the defining trait for all parallelogr