Simple plane closed figure bounded by three line segments is called is:
(a) Angle
(b) Triangle
(c) Circle
(d) Square

Hint:
Here, we need to complete the given statement with the correct option. We will use the definitions of an angle, triangle, circle, and a square to check which of the options is the correct option.

Complete step by step solution:
We will check the definitions of each of the options one by one to find the correct option.
An angle is a figure formed by two rays that have a common vertex.
For example: An angle \[\angle BAC\] is formed by the two rays \[\overrightarrow {AB} \] and \[\overrightarrow {AC} \], with the common vertex \[A\].
An angle is not a closed figure.
Therefore, option (a) is not the correct option.
A triangle is a closed figure that is formed by three line segments joined vertex to vertex.
For example: Triangle ABC is formed by the line segments AB, BC, and AC.
Thus, option (b) is the correct option.
We will also check the definition of option (c) and option (d).
A circle is a closed figure formed by taking a point as a centre, and drawing its locus at a constant distance, called the radius.
Line segments are always straight.
We know that a circle is not formed using any straight line.
Thus, option (c) is incorrect.
A square is a quadrilateral where all sides are equal, and the interior angles are right angles.
Since a square is a quadrilateral, it is a closed figure made from four line segments.
Thus, option (d) is not correct.
Therefore, we can say that only option (a) completes the given sentence and hence, it is the answer.

Note:
A plane closed figure constructed using straight lines is called a polygon. A polygon that has only three sides is called a triangle. A polygon that has only four sides is called a quadrilateral. A square is rather a special case of a quadrilateral. A circle is a closed figure, but it is not a polygon since it is not formed using straight lines.