Which solution should you implement?

You are the Office 365 administrator for your company.
The company requests that you implement a document collaboration and social networking solution
that meets the following requirements:
Users must be able to join groups to receive project updates.
Any user must be able to post an event.
You need to implement a solution.
Which solution should you implement?

A.
Microsoft SharePoint document libraries

B.
Microsoft Yammer

C.
Microsoft SkyDrive Pro

D.
Microsoft SharePoint newsfeeds