。
She declined the invitation.
Literal
She [topic-は] that invitation [object-を] declined.
断る is the everyday verb for politely refusing or declining — softer than 拒絶する ('reject') but firmer than the formal phrase お断りいたします. 招待 ('invitation') is a Sino-Japanese word, more formal than the casual お誘い ('invite'). The collocation 招待を断る is common in social and business contexts where one needs to convey having said no without elaborating on the reason.