This is the correct answer, since firewall does not mean "VPN" we have to select "Encrypted Virtual Private Networks". With a VPN and encryption we can provide secure communication in a transparent way for the users between the endpoints achieving "Confidentiality". This confidentiality is achieved through encryption, and this encryption relies on encryption algorithms like AES, DES, CAST and others. Screened Subnet are not related to secure data over public networks, it's a place to put our network services accessible from the outside. Digital certificates do not provide confidentiality, they only provide integrity.

Separation of duty is necessarily determined by conditions external to the computer system. The Clark-Wilson scheme includes the requirement that the system maintain the separation of duty requirement expressed in the access control triples. Enforcement is on a per-user basis, using the user ID from the access control triple.

This is one of the features of intrusion detections, instead of being pro-active, it has a reactive behavior. When we set an IDS system inside of our network or hosts, the
IDS agent is constantly monitoring in real time what activities are being performed in the infrastructure. If the IDS founds a malicious activity taking place it can take actions against it like disabling interfaces, alerting the administrators or sending network attacks to the source to put it out of service.
As a difference to the detective behavior of IDS, we can also increase the security with practices like hardening our systems ,this is considered a preventive practice.