Which of the following shapes have opposite sides equal in length?

The correct answer is a parallelogram. A parallelogram is defined by having opposite sides that are both equal in length and parallel. This characteristic is one of the key properties that distinguishes parallelograms from other quadrilaterals. For instance, in a rectangle (a specific type of parallelogram), both pairs of opposite sides are equal in length, but this applies broadly to all parallelograms.

In contrast, circles do not have sides or edges, as they are defined by a single continuous curve that maintains a consistent distance from a central point. Triangles, while they have three sides, do not have opposite sides since the concept of "opposite" applies to four-sided figures (quadrilaterals). Lastly, while a rhombus (which is a specific type of parallelogram) also has equal opposite sides, the broader category of parallelogram encompasses more shapes and is the correct general answer to the question posed. Therefore, the properties of the parallelogram specifically highlight the concept of opposite sides being equal, solidifying its correctness as the answer.