Universal Container Sales director started a new confidential project where he had to create a chatter group and invite the members that will be participating into this project. This chatter group should not be visible to non-members. It should only be accessed through invite. Which chatter group will the salesforce admin recommend for the sales director?

Answer options:

A.Public Group
B.Private Group
C.Unlisted Group
D.Listed Group

Correct Answer: C 
Unlisted chatter group offers a privacy more than private groups in chatter where users cannot ask to join. They can only join through invite. Unlisted groups need to be enabled by the salesforce admin to be able to use them. 
Option A is incorrect because public groups are available for everyone and anyone will be able to view and add posts or comments 
Option B is incorrect because private group are accessible through searching for it and people must ask to join it. 
Option D is incorrect because listed group is not a chatter group. 

https://help.salesforce.com/articleView?id=collab_group_creating.htm&type=5