You are a global admin in a company with a Microsoft 365 subscription. Your users have their mailboxes in Exchange online. You have no archiving solution and wish to enable mailbox archiving within Office 365 for all users. What should you do first?

Answer options:

A.In Exchange Online admin center, select Migration -> Batch
B.In Exchange Online admin center, select Organization -> Sharing
C.In Security & Compliance Center, select Information governance -> Archive
D.In Security & Compliance Center, select Information governance -> Retention

Correct Answer: C 
You can enable archive for your users by: 
Navigate to Security & Compliance Center, select Information governance, select Archive: 
Select all mailboxes by clicking on the first one in the list, holding down the Shift key, and then clicking the last one in the list. 
In the details pane, under Bulk Edit, click Enable. 
View the warning and click Enable to start the process. 

Option A is incorrect. This is related to migration workloads from exchange on-premise to exchange online.
Option B is incorrect. This is related to sharing configurations between yours and other organizations.
Option D is incorrect. This is related to setting up retention policies to determine what items to keep and for how long.
Reference: 
To know more about setting up mailbox archiving, please refer to the link below: 

https://docs.microsoft.com/en-us/microsoft-365/compliance/set-up-an-archive-and-deletion-policy-for-mailboxes?view=o365-worldwide