Mum told me he would be back -----.

应该选择C.
这道题目考的是直接引语转化成间接引语.
如果直接引语的时间状语是in two days,next week,tomorrow,
改成间接引语时,要变成in the next two days,the next week,the next day.
如果不是涉及这个,口语中用哪个都没问题.