。
She turned down his marriage proposal.
Literal
She [topic-は] his [genitive-の] marriage-request [object-を] declined.
求婚 ('proposal of marriage,' literally 'request for marriage') is a single Sino-Japanese word that compresses 結婚の申し込み into something more direct and slightly more formal. Built from 求 ('seek, request') + 婚 ('marriage'). Both phrasings are valid and idiomatic; 求婚 is somewhat more literary or romantic, 結婚の申し込み more everyday.