There are two correct answers to this question. The first is C, Chatter files. This is a process, which allows the user to do things in the cloud, like uploading, sharing, and following data. They are useful is allowing users to talk about the data, regardless of the device they are using.
It also helps eliminate more emails and keeps a record of all the comments about the file for easy future access. The other correct answer is D, libraries. This is where information can be shared and stored. Permissions can be given only to give specific people access to the library.