Based on the "Defense in Depth" model in cybersecurity, please categorize the following as to whether they relate to: Application, Presentation, Session, Transport, Network, Data Link, or Physical.

$1.$ A criminal hacker tailgates an employee through an exterior door into a secured facility, explaining that they forgot their badge at home.

$2.$ A zero$-$day goes undetected by antivirus software.

$3.$ A criminal successfully gains access to HR$$s database.

$4.$ A criminal hacker exploits a vulnerability within an operating system.

$5.$ A hacktivist organization successfully performs a DDoS attack, taking down a government website.

$6.$ Data is classified at the wrong classification level.

$7.$ A state$-$sponsored hacker group successfully firewalked an organization to produce a list of active services on an email server.