Hi! There are many many ways how you can structure costs. You can do it according to business lines (for example interest vs non interest), business processes, functional groups, logical categories (e.g., fixed and variable), or many other. There is no "best" option! You have to just be aware of the fact that there are many possibilities, and that it is your task to find out, which kind of categorization makes most sense given the context and data available. This is done by means of clear communication with the interviewer.