what is the very simple sculpture in world?

Answer 1

I'd say the oval. It is ultimately the universal shape of the universe. Look at the planets - they are spherical. And their orbits elliptical.

The universe is probably oval itself. It certainly isn't going to have sharp corners, not from what we gather. Plus not many things in nature are perfectly round.

Answer 2

snowmen.

snowmen are work of sculptures, even if they are only temporary. ^_~