The files in the /etc/skel directory are used by the…

A.
pwconv command

B.
pwunconv command

C.
useradd command

D.
passwd command

Explanation/Reference:
useradd uses the config-file /etc/default/useradd which contains the shell variable $SKEL, that points to /etc/skel (if uncommented). useradd copies all files and directories in $SKEL to the new home directory.