Definition: If $r$ and $s$ are statements, $r$ unless $s$ means if $s$ then $r.$

Consider the following statement.

This door will not open unless a security code is entered.

Which of the following expresses the statement in if$-$then form?

If a security code is entered, then the door will not open.

If the door will not open, then a security code was entered.

If a security code is not entered, then the door will open.

If a security code is not entered, then the door will not open.

If the door will open, then a security code was not entered.