Correct Answer: To determine the correct built-in user group for each user, we need to evaluate the privilege level (priv_lvl) provided in the authentication logs. The logs show the following privilege levels for each user: User 1 has priv_lvl=0, User 2 has priv_lvl=1, User 3 has priv_lvl=19, and User 4 has priv_lvl=15. Here is how these privilege levels typically map to built-in user groups: priv_lvl 0 indicates no built-in user group, priv_lvl 1 indicates operators, priv_lvl 15 indicates administrators, and priv_lvl 19 indicates auditors. Therefore, the correct assignments are as follows: User 1 -> no built-in user group, User 2 -> operators, User 3 -> auditors, User 4 -> administrators.