How are the sides of a rhombus defined?

The definition of a rhombus is characterized by specific properties, primarily involving its sides and angles. A rhombus is a type of quadrilateral where opposite sides are parallel, which is a defining feature of all parallelograms. Additionally, in a rhombus, not only are opposite sides parallel, but all sides have equal length, providing a unique symmetry.

Moreover, the angles in a rhombus show a particular relationship: opposite angles are equal, which supports the property that when two pairs of opposite angles are equal, it reflects the shape's symmetrical nature. This characteristic of being both equal and opposite means that if one angle measures, for example, 60 degrees, its opposite will also measure 60 degrees, while the adjacent angles will measure 120 degrees each—always summing to 360 degrees.

Thus, the correct choice encapsulates these essential properties: it highlights that both opposite sides are parallel, equal, and opposite angles are equal, which collectively define a rhombus. This precise definition is crucial for identifying and understanding the unique properties of a rhombus in geometry.