。
She passed the exam easily.
Literal
She [topic-は] easily exam [target-に] passed.
簡単に is the adverbial form of the na-adjective 簡単 ('easy / simple'), formed by adding に to the stem — the regular pattern for turning na-adjectives into adverbs. 試験に合格する ('pass an exam') uses に (not を) for the exam, because 合格 ('pass') frames the exam as a target reached rather than an object acted upon — like 大学に入る ('enter university'). This に-of-target pattern recurs across Japanese with verbs of attaining or reaching. Compared to the casual 試験に受かる, 合格する is slightly more formal — the kind of word you'd see on a results notification.