Why only triangular shapes are used in constructing bridges, towers and building?

Because they maintain their shape and angle when pressure or force is applied to any one of their vertices, triangular shapes are an ideal choice for bridges, towers, and buildings. Other shapes, such as squares, pentagons, hexagons, etc., aren't as well suited for building since pressure given to any one of their vertices causes them to eventually change shape. Therefore, using such shapes will be costly in terms of both lives and money.

Hence, the correct answer is option B.