Which statement about users and user groups is correct?

A group can contain any number of users as members; there is no one-user limit imposed by the group model.

Linux systems routinely have dozens or hundreds of groups (wheel, sudo, audio, video, etc.) in addition to user-private groups.

Every user has a mandatory primary group recorded in /etc/passwd; group membership cannot be absent.

When a user account is created with useradd, a primary group is always assigned - either an existing group specified with -g or a new group with the same name as the user (the user private group scheme). This primary group is stored as the GID field in /etc/passwd, so it is impossible for a user to exist without group membership.