Attackers exfiltrate data using steganography embedded in images.

CEH v13 explains that steganography-based exfiltration hides data within benign-looking files, making it extremely difficult to detect via firewalls or IPS alone. Blocking all outbound traffic is impractical, and IPS systems are not designed to analyze file content deeply for hidden data.

The most effective countermeasure is steganalysis, which involves inspecting files for statistical anomalies, altered pixel distributions, or hidden payload patterns. CEH v13 identifies steganalysis tools as the only reliable method to detect and decode hidden data.

Traffic monitoring (Option C) helps identify suspicious transfers but cannot confirm steganography. Therefore, Option D is correct.