。
She decided on a red jacket.
Literal
She [topic-は] red jacket [on-に] decided.
~に決める (きめる) means 'to decide on' — the に marks what was chosen. This is the standard pattern for finalizing a choice: レストランに決める ('decide on a restaurant'), 赤に決める ('go with red'). 上着 (うわぎ) means 'jacket' or 'outer garment' — literally 上 (upper) + 着 (garment). Contrast with 決まる (intransitive: 'to be decided'), where the decision happens on its own.