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

`/etc/pgroup` is not a standard file for managing primary groups in Linux.

`/etc/shadow` stores encrypted passwords and password expiration information, not primary group assignment.

`/etc/group` defines group names and their members (supplementary groups), but the primary group assignment for a user is listed in `/etc/passwd`.

The `/etc/passwd` file stores user account information, and the fourth field in each user's entry explicitly specifies the numeric Group ID (GID) of their primary group, linking the user to their initial group membership.

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