Which one of the following is different from the remaining three?

The classification of geometric shapes often distinguishes between polygons and non-polygons. A triangle is a polygon, defined as a plane shape consisting of three straight line segments. Similarly, a square is a four-sided polygon (quadrilateral) with straight sides. In contrast, circles and ellipses are non-polygons because they consist of curved lines rather than straight segments. However, in the context of this specific set, the triangle is the only shape that can be considered 'different' based on its unique property of being the simplest polygon with the minimum number of sides (three), whereas the others are either quadrilaterals or curved figures. Alternatively, if the logic follows the number of sides, the triangle is the only one with an odd number of vertices/sides. Given standard logical reasoning patterns for such questions, the triangle is often the outlier as the most basic polygonal unit.