Explanation

The triangle is the intended odd one out: it is the only figure with three sides, the simplest polygon and the only one with an odd number of vertices, whereas the square is a quadrilateral and the circle and ellipse are curved conic figures. Two reputable solved keys mark (A). Note: one published key gives (D) (ellipse), since the ellipse is the only non-uniform curve, but the weight of key evidence supports (A).