。
She prepared lunch.
Literal
She [topic-は] lunch [possessive-の] preparation [object-を] did.
昼食の用意をした — 用意 ('preparation') paired with する is the standard expression for getting something ready. Both 用意 and the near-synonym 準備 translate as 'preparation,' but they have a slight nuance difference: 用意 is more general — getting things ready in a basic sense — while 準備 implies organized, step-by-step readying. Either could fit a meal, but 用意 fits the casual, everyday tone here.