The correct answer is C. Firewalls and IPS systems protect the perimeter, but they do not replace system hardening, access control, patching, auditing, and regular security testing on the internal network elements themselves. In CEH security concepts, defense should be implemented in layers, commonly called defense-in-depth. This means that even if an attacker bypasses perimeter defenses, internal systems should still be protected.
Because these network elements are Linux-based computers, they require strong user account management, secure authentication, least privilege, service hardening, vulnerability assessment, patch management, logging, and periodic audits. Strong passwords and unique user IDs also support accountability and reduce the risk of unauthorized access.
Option A is incorrect because relying only on firewalls and IPS creates a single point of failure. Option B mentions disaster recovery but does not address preventive security. Option D is also incomplete because physical security alone cannot stop remote attacks, malware, insider threats, misconfigurations, or credential abuse.
Therefore, the best policy is to harden the systems and perform regular security testing and audits.
