The answer to this is B. A composite data flow is a data flow that is composed of other data flows. This can be very useful especially when there are various processes that are part of specific data flows that need to be described to come up with a comprehensive data flow that can be checked by people within the team.
Take note that there are certain processes that will not work well if the input flows will not be available. To make everything clearer, you can split one flow or make sure that you can merge two different flows into one to create one understandable data flow.