I have been reading this for days, so I will comment now. The fact that the manager is making a Decision on the most appropriate solution indicates that she has infact exercised a decisional role. Now given, the word choice was Decision-Making, Mintzberg's Roles of management (as shown in Business Studies in Action the text book) display Decisional as Decision Making
Once again Board Of studies has been ambiguous by their nature, trying to deceive students rather than test what you actually know.
I believe the answer was intended to be Decision Making seeing as though they emphasised the word selecting. However C is just as feasible a choice.
I think they will make both correct