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She looks more beautiful wearing red.
Literal
She [topic-は] red clothes [object-を] wore [alternative-方が] beautifully looks.
~た方が sets up a comparison between alternatives: 'the option of wearing red' vs an implied other option. 方 (ほう) means 'side' or 'alternative,' and が marks it as the subject of 見える ('looks/appears'). 美しく is the adverbial form of 美しい, modifying 見える. The overall pattern says: 'comparing alternatives, the red-wearing one looks more beautiful.'