What defines a parallelogram?

A parallelogram is defined by the property that its opposite sides are parallel and equal in length. This geometric figure is characterized not just by the equal length of its sides, but also by having pairs of sides that run parallel to each other. This fundamental property ensures that opposite angles are also equal, and the sum of adjacent angles equals 180 degrees.

While it is true that a rectangle and a square, which are types of parallelograms, have right angles and equal sides respectively, not all parallelograms meet those criteria. Therefore, defining a parallelogram only by equal angles or equal sides doesn't encompass the full range of parallelograms, which can include figures with varying angles and side lengths, as long as the opposite sides remain parallel and equal.

These properties make option B the most accurate representation of what defines a parallelogram. Other statements describe specific types of quadrilaterals but do not capture the essence of a parallelogram as a broader category.