Question

which shape is consider to be the most rigid shape

Answer 1

One would classify the triangle as a rigid shape because none of its edgelengths or angles would change because of the intrinsic shape of the object. The square on the other hand isn't geometrically limited to having 90 degree angles when you apply pressure to it, so it can change into a rhombus rather quickly.

Answer 2

triangle is considered as the most rigid