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Pyramid i guess.
I was wondering why ?
same here. lol
And by stable you mean? Could be a gömböc (google it)
The tetrahedron is one, and an equally stable object in 3D is a sphere whose material , even if flexible, does not stretch or shrink in response to compressive or tensile loads.
... is a spherical shell ...
The term "stable" is still pretty vague. What do you mean by it? Something that remains still? A wing is actually pretty stable when you're flying, too! Be precise!
Perhaps a better term would be "rigid"'
With that in mind, suppose you have six beams, all the same length. You form a ragular tetrahedton with the six beams. The result is rigidity, even if the joints are flexible. Suppose you can form the surface of a sphere using flexible material thae does not stretch or shrink shen subjected to compressive or tensile forces. That shell is also rigid.