Answer Posted / alf55

Answer 2 is the most correct. Really "su" is "switch user"
and its format is "su optional_options optional_username"
where a missing "username" defaults to "root".

The options "-", "-l", and "--login" are all equivalent.

When the "-" is used, the shell is initialized as if logged
into the machine as the specified user (hence you also end
up in your the home directory of "username". Without the
"-", the current environment is left intact but the user has
been switched to that of "username" hence you are still in
the current directory.

From the "man su" page:
{{{
-, -l, --login
Provide an environment similar to what the user
would expect had
the user logged in directly.

When - is used, it must be specified as the last
su option. The
other forms (-l and --login) do not have this
restriction.
}}}

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