The correct answer is C. Application sensor set to monitor on all the FortiGate devices .

The study guide clearly explains that application control is the feature used to identify OT protocols. It states that “application control detects the protocols used in applications like Modbus, IEC 104, and the contents of the telecontrol messages” and also says “You can use application control signatures to detect OT protocols.” It further shows an example where a Modbus application control profile is enabled on a firewall policy “for OT protocol visibility in the monitor status.” This directly matches the requirement in the question, which is to detect OT protocols and increase traffic visibility .

The other options do not fit the requirement as precisely. Device detection is for identifying devices and collecting endpoint information, not for detecting industrial protocols. Inline IDS and IPS are focused more on detecting or blocking attacks, exploits, protocol abnormalities, and known vulnerabilities. While IPS can inspect some OT traffic, the study guide distinguishes it from application control by stating that IPS signatures tend to detect exploits, whereas application control signatures tend to provide protocol detection at various levels . Therefore, the required security sensor for OT protocol detection and traffic visibility is the application sensor in monitor mode .