The answer to this is D. All of the above statements are true. Activity Diagram is one of the diagrams that are used when people would like to point out the different aspects of the system. In order to do this, you would need to create something that is similar to a flowchart.
The flowchart will show how one activity can connect with and can affect the next activity that will be done. There are some people who use this because they know how it can benefit their company when they know the various activities that should be done.
Knowing the operation of the system can also help people plan ahead regarding the things that they should do to keep everything smooth sailing.