楼主正确.不好意思.差点犯了愚蠢错误.
确实修饰tell.
You must tell us ---------what you would like to do.(exact)
不填也正确
但是有选项exact,exact是形容词
既然整句正确,自然只能选择修饰动词tell的副词.
exact的副词是
exactly
You must tell us --exactly-------what you would like to do.
你必须明确地告诉我们你到底要做什么.