What were you so upset about?
My son is not passing exams upset me most.what for 不是一个固定搭配 吗？what were your so upset for?
My son is not passing exams upset me most.
两个语法上都可以，我个人感觉口语上，用 upset for 比较少。What about, what for 都可以是固定搭配，看动词搭配那个更通顺。