We don’t know it next. Let’s go and ask Mr Li.A what to do B to do what C whether to do D

We don’t know it next. Let’s go and ask Mr Li.
A what to do B to do what C whether to do D to do whether

解析：动词不定式作定语时要放在所修饰词的后面，whether to do是如何去做的意思，what to do是做什么的意思。It是我们做的对象，我们应是不知道是否去做，所以选C.

感谢回答，我学习了