A quadrilateral is a four-sided polygon. Among the shapes listed, a rhombus is a type of quadrilateral. Here's a breakdown:
1. **Rhombus**: A rhombus is a quadrilateral with all four sides of equal length. It is a special type of parallelogram where opposite sides are parallel and opposite angles are equal.
2. **Triangle**: A triangle is not a quadrilateral as it has only three sides.
3. **Circle**: A circle is not a quadrilateral; it is a two-dimensional shape with all points equidistant from the center.
4. **Hexagon**: A hexagon is also not a quadrilateral. It is a six-sided polygon.
Therefore, among the options provided, a rhombus is the shape that is a quadrilateral.
