。
She accepted his marriage proposal.
Literal
She [topic-は] his [genitive-の] proposal [to-に] responded.
応じる ('respond, comply, accept') is a slightly formal verb covering responses to invitations, requests, demands, and proposals — implies considered acceptance rather than enthusiastic embrace. The に marks what's being responded to. Compare with the slightly more dramatic 受け入れる ('accept, take in') and the colder 承知する ('acknowledge, agree to').