Answer:
Elegant, refined.
Explanation:
The sentence reads -
Plentiful is to abundant as polished is to refined / shiny / elegant.
The synonym of plentiful is abundant, ample, rich etc, having a positive connotation in the use of the word. Likewise, the synonym for polished will be refined, shiny or elegant. This word also have a positive connotation in it's use. Polished means to be experienced, or something elegant, far beyond normal appearance.
