。
She turned down his marriage proposal.
Literal
She [topic-は] his [genitive-の] marriage [genitive-の] proposal [object-を] declined.
結婚の申し込み ('marriage proposal,' literally 'application for marriage') uses the everyday word 申し込み ('application, registration, proposal') — same word used for job applications, course enrollment, and the like. The choice over 求婚 ('proposal of marriage,' more direct) gives a slightly oblique, formal-feeling phrasing. 断る ('decline, refuse') is the everyday refusal verb.