Which command removes all permissions for others on a file named file1?

• A. setfacl -m o::r file1
• B. setfacl --remove-all file1
• C. setfacl -m o::- file1
• D. setfacl -m o::-f file1

Answer: (C)

ACLs let you assign permissions to different classes of users, including others (the world). To remove all access for others, you modify the ACL entry for the others class to have no permissions. In setfacl syntax, that is done with o:: followed by an empty permission set, using -m to modify. So the command sets the "other" entry to none for file1, removing all permissions for others while leaving user/group ACLs and other entries unchanged. This is precisely what clears all access for others. The other approaches either grant specific permissions (not remove), or use an invalid or unrelated option.