Susan is working with an IT company. She has been asked to make sure that all of the functions in the company are in place and performing optimally. Which management function should Susan employ to do this?

According to Henri Fayol, it can be said that there are five functions of management: planning, organizing, staffing, directing, and controlling. From the description in the question, it seems that Susan should employ the controlling management function - which is defined as ensuring that all the ongoing process are proceeding as planned.

It is clear that this definition is precisely what Susan needs to do to fulfill the request that she was given.