。
She suggested throwing a party.
Literal
She [topic-は] party [object-を] hold [the act-こと] [object-を] proposed.
パーティーを開く is the standard idiom for 'to throw / host a party,' using 開く ('open') in its event-holding sense — the same verb covers 会議を開く ('hold a meeting'), 展覧会を開く ('hold an exhibition'). The full clause is then nominalised with こと to feed it into 提案する ('propose, suggest'); ことを提案する is a stock pattern for proposing an action. The の/こと nominaliser distinction matters here — こと is preferred over の for abstract proposals and decisions because the proposal is about the concept of holding a party rather than a specific witnessed event.