。
She came home very late.
Literal
She [topic-は] very late returned-home.
遅く is the adverbial form of the i-adjective 遅い ('late, slow'). 帰宅する is the Sino-Japanese 'return home' compound (帰 + 宅, return + dwelling), more formal and clinical than the everyday 帰る — common in news, official records, and any context that wants neutral distance. A train station announcement would say 帰宅; a friend texting you would say 帰る.