The correct answer is Test the theory, which aligns directly with the CompTIA Network+ N10-009 troubleshooting methodology. According to the official troubleshooting steps, once a problem has been identified and relevant information gathered, the next step is to establish a theory of probable cause and then test that theory to determine whether it is valid.
In this scenario, the engineer has already completed the problem identification phase by confirming that all users are experiencing latency and correlating the issue with the installation of a new switch. The engineer has also formed a theory: that a network loop caused by the lower bridge ID of the new switch may be impacting Spanning Tree Protocol (STP), resulting in congestion and latency. Because this theory has already been developed, the next logical and required step is to test the theory-for example, by reviewing STP states, temporarily disabling ports, or adjusting bridge priorities.
Implementing a solution would be premature without validating the theory, and verifying full system functionality occurs only after a fix has been applied. The Network+ objectives stress following a structured methodology to avoid unnecessary changes and minimize network disruption. Testing the theory ensures accuracy before corrective action is taken, reducing risk and downtime.