The correct answer is A. Utilizing port mirroring on a firewall appliance

The key issue is that the tester must avoid tampering with the ICS host because modifying it could void the manufacturer’s support terms. Port mirroring, also known as SPAN, allows the tester to passively copy network traffic from a switch or firewall interface to a monitoring port. The tester can then inspect the copied traffic from a separate analysis system without installing software, changing configurations, or interfering with the ICS application.

This is the most effective and safest method to determine whether communications are encrypted in transit because the assessment team can observe whether traffic is plaintext or encrypted while avoiding changes to the protected host.

B is incorrect because installing packet capture software on the server modifies the host and could violate the support terms.

C is incorrect because reconfiguring the application to use a proxy changes the application behavior and would be considered tampering.

D is incorrect because requesting certificate pinning be disabled changes the application’s security configuration and is unnecessary for simply validating whether traffic is encrypted in transit.

In PenTest+ terms, this falls under Information Gathering and Vulnerability Scanning, especially passive traffic analysis, packet capture, and safe testing considerations in sensitive environments such as ICS/OT.