Which of the following is a property of a parallelogram?

A property of a parallelogram is that opposite sides are equal in length and parallel. This characteristic is fundamental to the definition of a parallelogram. In such a figure, not only are the opposite sides equal in length, but they are also configured in a way that makes them parallel to each other, resulting in two pairs of opposite sides each having these properties.

The distinctions of other shapes, such as rectangles or squares, might include right angles or equal diagonals, but these do not define a parallelogram itself. Therefore, while it's true that rectangles (a specific type of parallelogram) feature all right angles and equal diagonals, the defining characteristic of a parallelogram is the equality and parallelism of opposite sides. This makes the property of opposite sides being equal and parallel the most accurate descriptor of a general parallelogram.