CEH v13 identifies encrypted communication channels as one of the most common and effective firewall evasion techniques. Firewalls that rely on packet inspection or signature-based filtering often cannot inspect encrypted payloads without SSL/TLS interception capabilities.

By encrypting malicious traffic—using HTTPS, VPN tunnels, or encrypted C2 channels—attackers can bypass firewall rules that inspect packet contents. CEH v13 emphasizes that this technique is widely used in malware communication, data exfiltration, and command-and-control operations.

IP spoofing (Option A) is limited by ingress and egress filtering and is less effective against modern firewalls. Open-source operating systems (Option B) do not inherently evade firewalls. Social engineering (Option D) targets users, not firewalls.

Therefore, Option C is the correct and CEH-aligned answer.