Which of the following defines a rhombus?

The defining characteristic of a rhombus is that all four sides are congruent, meaning they are of equal length. Additionally, the diagonals of a rhombus have the specific property of being perpendicular to each other, intersecting at right angles. This combination of properties differentiates the rhombus from other quadrilaterals.

While it is true that a rhombus has opposite sides that are parallel and equal, which is a characteristic mentioned in another option, the defining properties that distinguish a rhombus most clearly include the equality of the sides and the right angles formed by the intersecting diagonals. Hence, noting that the diagonals are also perpendicular helps solidify the understanding of the structure of a rhombus compared to other quadrilaterals.

Overall, option C encapsulates the key features that confirm a shape is definitively a rhombus.