Explanation:

- Parallelogram: A quadrilateral with opposite sides parallel. It's a type of polygon.

- Hexagon: A six-sided polygon. It can be regular (all sides and angles equal) or irregular.

- Rhombus: A four-sided polygon (quadrilateral) with all sides of equal length. It’s a special type of parallelogram.

- Ellipse: A curved shape, resembling an elongated circle, but not a polygon.

The first three options are polygons with straight sides, while the ellipse is not. It is defined by its curved shape, making it different from the others.

Ellipse is the odd one out.

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