The correct answer is option 1, Triangle. The triangle is different from the remaining three options (square, circle, and ellipse) because it is the only option that does not have four sides.

A triangle is a polygon with three sides and three angles. It is a closed figure with three straight sides joined together. It is typically represented by drawing three lines connected to form a shape with three angles.

On the other hand, both the square and ellipse have four sides. A square is a polygon with four equal sides and four right angles. It is a closed figure that has all sides of equal length and all angles measuring 90 degrees.

Similarly, an ellipse is a closed curve that is symmetric about its center. It is elongated and resembles a flattened circle. However, it still has the characteristic of having four sides, unlike the triangle.

The circle is also different from the triangle, but for a different reason. A circle is a two-dimensional shape that has no sides or angles. It is defined as the set of all points in a plane that are equidistant from a fixed center point. It can be thought of as a perfectly round shape.

Therefore, the correct answer is option 1, Triangle, as it is the only option that does not have