The correct answer is iii. circle.
A circle is defined as the set of all points in a plane that are at a fixed distance, known as the radius, from a central point, which is called the center. Each point on the circle is equidistant from the center, forming a perfect round shape. In contrast, the other options do not describe this concept:
- i. collinear: This refers to points that lie on the same straight line.
- ii. angle: An angle is formed by two rays that share a common endpoint, which does not involve a fixed distance from a single point.
- iv. coplanar: This describes points that lie within the same plane but does not imply a fixed distance from a point.
Therefore, the best answer is indeed a circle.