Which of the following files assigns a user to its primary group?

/etc/pgroup is not a standard Linux file used for primary group assignment.

The /etc/shadow file stores encrypted user passwords and password aging information, not primary group assignments.

The /etc/group file lists groups and their members (supplementary groups), but the primary group assignment for a user is defined in /etc/passwd.

The /etc/passwd file stores user account details, with the fourth field (GID) indicating the numeric ID of the user's primary group. When a user logs in, this GID determines their initial group membership and default file creation permissions.

The /etc/gshadow file stores encrypted group passwords and group administration information, similar to /etc/shadow for users.