The correct way to rewrite this phrase is the daisy's center.
The original sentence uses a possessive structure which can be better written with the use of 's, The daisy's center.
The daisys' center has a spelling mistake, in order to talk about a daisy in plural, we should use the word daisies instead of daisys.
The daisies center is well written but it has a plural noun and in the original sentence we were introduced to a singular noun.
