Correct Answer - Option 3 : 170

__Concept:__

A polygon made by joining n non-collinear points will have n sides and (^{n}C_{2} - n) diagonals.

__Calculation:__

The polygon has n = 20 sides.

The number of diagonals will be: (20C2 - 20)

= \(\frac{20!}{2!\times18!}-20\)

= **170**.