Which of the following is used to group subscriptions together?

Answer options:

A.Resource groups
B.Region pairs
C.Management Groups
D.Availability sets

Answer : C 
Management groups is used to organize subscriptions together. You can then manage access and compliance of the subscriptions that are part of the management group.
Option A is incorrect because this is used to logical group resources
Option B is incorrect because this is used by Azure to pair regions together
Option D is incorrect because this is used to manage the availability of virtual machines
For more information on management groups, please visit the below URL 

https://docs.microsoft.com/en-us/azure/governance/management-groups/overview