You didn't provide the list of choices, but I can tell you some things that were seen as bases of the classical conventions of beauty.
Geometry was one such source -- seeing beauty in symmetry and consistency of shapes.
In a similar way, astronomy even provided a source, as those who looked out to the heavens saw what they referred to as the "music of the spheres," the way that planetary bodies all moved in harmony as a system.
The human form was a very strong model for determining classical beauty, and artists did much study and depiction of the human form.