。
I'm sure she'll pass the exam.
Literal
She [topic-は] surely exam [in-に] will-pass [probably-でしょう].
試験に合格する ('pass an exam') uses に for the exam being passed — a common pattern with 'achieving' verbs (試験に合格する, 大学に合格する, 目標に達する). Compare 試験を受ける ('take an exam'), where the exam is the direct object of 'taking.' The に here is the goal/target marker. The polite ~でしょう gives the prediction a measured, somewhat formal flavor; だろう would work too but feels slightly more casual.