I assume you mean question 5 of the hybrid exam (which is a selection of questions from September 2015 and December 2015).
There is no point in my typing our an entire answer, because you have the answer from the ACCA website, and so it is better that you ask about which bit of the answer you are not clear about.
I assume that you have watched my free lectures on divisional performance measurement, in which I explain what is meant by ROI and how we calculate it?