Final answer:
The HR Admin needs the User Admin [user_admin] role to remove users from groups, as it provides comprehensive user management permissions.
Step-by-step explanation:
The role required for an HR Admin to remove users from groups, in addition to the sn_hr_core.admin role, is the User Admin [user_admin] role. The User Admin role has the permissions necessary to manage users comprehensively, which includes adding and removing members from groups as well as other user-related administrative tasks. Other roles listed such as User Writer, User Reader, and HR User Admin do not provide the same level of privileges for this specific function.