The correct answer is A. Port mirroring

Port mirroring, also known as SPAN, allows network traffic from one or more switch ports to be copied to a monitoring port. This lets the tester passively observe network communications without directly interacting with production systems.

This is especially appropriate in sensitive environments such as ICS, SCADA, OT, and power plant control networks, where active scanning or probing may disrupt operations. Since the tester needs to identify vulnerabilities observable in the network stack while following rules that prohibit interaction with production systems, passive traffic capture through port mirroring is the safest and most appropriate technique.

B is incorrect because storyboarding is a planning or communication technique and does not observe network-stack behavior.

C is incorrect because a write blocker is used in forensic investigations to prevent modification of storage media. It does not help assess network-stack vulnerabilities.

D is incorrect because a SAST tool analyzes source code for vulnerabilities. It does not passively inspect live network traffic or network-stack behavior.

In PenTest+ terms, this falls under Information Gathering and Vulnerability Scanning, specifically passive reconnaissance, network traffic analysis, and safe testing practices in ICS/OT environments.