Your Answer is

Correct Answer is H

Explanation

For polygons, a diagonal is a line from a point to a vertex of a non-adjacent point, and two repeated diagonals count as one when counting how many diagonals there are.

For the octagon given in the title, the diagonal from the first vertex is 8 - 1 (self) - 2 (two points adjacent to the first vertex) = 5 bars, and the diagonal of the second vertex is also 8 - 1 - 2 = 5 bars. Since it cannot be repeated, from the third vertex to the sixth vertex, the diagonal line is reduced by one, that is: 4 lines at the third vertex, 3 lines at the fourth vertex, 2 lines at the fifth vertex, and 1 line at the sixth vertex. The total number of diagonal lines is: 5 + 5 + 4 + 3 +2 +1 = 20 lines.