Answer: I would contend that the right answer is the 3) "direct us."
Explanation: Just to elaborate a little on the answer, it can be added that an assignment is a specific task given to someone. The verb "direct" sounds vague and imprecise in this context, and particularly very harsh, since it suggests that the students have been given an order, rather than a task that is supposed to be enjoyable and meaningful for their course of studies. I would replace "direct us" for "asks us" or "tasks us with learning."