You have to implement an Azure DevOps strategy for the usage of mobile devices with App Center. 
You have to use distribution groups to control the relevant access to releases. 
Below are the key requirements for the various groups.

Name	Requirement

testgrpA

Application testers would be invited by email

testgrpB

These are early release users who use unauthenticated public links

grpC	These are application testers for all apps of the company

Which of the following type or group would you use for testgrpA?

Answer options:

A.Private
B.Primary
C.Public
D.Shared

Answer – A 
Here you would use Private Distribution groups. 
The Microsoft documentation mentions the following. 

Since this is clearly given in the Microsoft documentation, all other options are incorrect. 
For more information on App Center distribution groups, please visit the following URL- 

https://docs.microsoft.com/en-us/appcenter/distribution/groups