。
She did it on purpose.
Literal
She [topic-は] that [object-を] [on purpose-故意に] did.
故意 (koi) is the formal/legal Sino-Japanese term for 'intent, deliberate purpose, malice aforethought,' contrasting with 過失 ('negligence, unintentional fault'). The pair 故意 vs 過失 is a foundational distinction in Japanese criminal and civil law. As an adverbial, 故意に ('intentionally, on purpose') has a slightly weighty, blame-implying tone — softer than 'with malice' but heavier than the casual わざと ('on purpose, deliberately'). 行う (formal 'do') matches the elevated register; in casual speech you'd more often hear わざとやった.