A line segment connecting two opposite vertices of a polygon is called a .................
A. vertices
B. edge
C. segment
D. diagonal

Hint: To solve this question, we need to know the basic theory and properties related to the polygon. As we know A Polygon is a closed figure made up of lines segments (not curves) in two-dimensions. Here we will discuss the vertices, edge, segment, diagonal of a polygon and fill appropriate answers.

Complete step-by-step answer:
A vertex is a corner where edges meet as shown in the figure.
An edge is where two faces meet.

In geometry, a line segment is a part of a line that is bounded by two distinct endpoints, and contains every point on the line between its endpoints.
A closed line segment includes both endpoints, while an open line segment excludes both endpoints.

A diagonal is a straight line connecting the opposite corners of a polygon through its vertex. So, diagonal is a line segment connecting two non-adjacent vertices of a polygon. It joins the vertices of a polygon excluding the edges of the figure. The following shapes have a diagonal drawn on them:

The segment joining the two opposite vertices in a polygon is called a diagonal. d is a diagonal of polygon as shown in above figure.

Therefore, (D) is the correct answer.

Note: A minimum of three-line segments are required for making a closed figure, thus a polygon with a minimum of three sides is known as Triangle. And also remember that any polygon has as many vertices as it has sides. Each corner has a certain measure of angles. These angles are categorized into two types namely interior and exterior angles of a polygon.
The sum of all the interior angles of a simple polygon having n-sides = $\left( {n - 2} \right) \times {180^0}$