。
She still hasn't accepted his marriage proposal.
Literal
[topic-は] still his marriage [genitive-の] application [object-を] has-not-accepted.
結婚の申し込み ('marriage proposal') is the formal phrase, with 申し込み ('application, formal request') used for the specific act of asking — also seen in 申し込みをする ('apply, submit a request'), 申し込み書 ('application form'). The same noun 申し込み generalizes to other formal requests (jobs, exams, contracts), giving 結婚の申し込み a slightly bureaucratic cast that contrasts with the modern colloquial プロポーズ ('propose,' from English). 受け入れる ('accept, take in') is a compound verb: 受ける ('receive') + 入れる ('put in, take in'). The negative resulting state 受け入れていない gives 'has not accepted.'