Which of the following commands will lock the user foobar’s account?

A.
userdel -r foobar

B.
moduser -l foobar

C.
usermod -L foobar

D.
userconf -l foobar

Explanation:
From the man pages:
usermod – modify a user account
-L, –lock Lock a users password. This puts a ! in front of the encrypted password, effectively disabling the password. You cant use this option with -p or -U.