A few questions as I am doing a brief Fundamentals Linux course

Hi all could any one please explain to me the answer and also why this is so on some questions below.

You want kelly, trent, and lisa to be members of the group managers (GID 501).

How would you edit the managers entry in /etc/group to accomplish this goal?

managers:501:kelly trent lisa

managers:501:kelly:trent:lisa

managers:x:501:kelly:trent:lisa

managers:x:501:kelly,trent,lisa

managers:501:x:kelly\trent\lisa

Also this please as well

What is wrong with the following /etc/passwd file entry?

4kelly:x:1043:Kelly Bawson:/home/myhome:/bin/passwd

Select two answers:

The default shell is set to /bin/passwd, which is an invalid shell.

The username is invalid; Linux usernames can't begin with a number

The home directory doesn't match the username

Either the UID or the GID field is missing

The encrypted password is missing

and one more please

Which service is used to provide a more secure method to authenticate users in Linux?

PAM

LDAP

SSH

Shadow

thank you in advance for any answers and help as I am new I just a little rattled with it but the more I practice the better I will get.

Kind Regards
Fazzer1980