。
She's worried about the exam.
Literal
She [topic-は] exam [of-の] thing/matter [object-を] is-worrying.
The pattern Xのこと ('the matter of X') is a stock way to package a topic as the object of a mental-state verb. 試験を心配している would also be grammatical, but adding のこと softens and broadens the focus from the exam itself to everything surrounding it — preparation, results, anxiety, the whole situation. This のこと frame is especially common with verbs of feeling and thought: ~のことが好き ('be fond of X'), ~のことを考える ('think about X'), ~のことを忘れる ('forget about X').