The business analyst should gather testing requirements from the stakeholder to address the need for user acceptance testing (UAT) for a particular story involving an update to an important trigger. UAT is end-user testing performed in a sandbox or test environment to verify that a project or enhancement works as intended and what was originally requested is actually being delivered. The business analyst should gather testing requirements from the stakeholder to understand what scenarios or cases need to be tested, what data or inputs need to be used, what outputs or results need to be expected, and what criteria or metrics need to be measured. The business analyst should also document and communicate these testing requirements to the testers and developers involved in UAT. Running all Apex tests using change set validation is not an action that the business analyst should take to address the need for UAT for a particular story involving an update to an important trigger. Running all Apex tests using change set validation is a process that checks whether deploying a change set would succeed or fail in another org without actually deploying it. Running all Apex tests using change set validation helps ensure code quality and coverage but does not verify whether a project or enhancement meets user needs or expectations. Performing testing on all objects in the trigger is not an action that the business analyst should take to address the need for UAT for a particular story involving an update to an important trigger. Performing testing on all objects in the trigger is a process that checks whether the trigger works correctly and consistently for all objects that it affects. Performing testing on all objects in the trigger helps ensure functionality and performance but does not verify whether a project or enhancement meets user needs or expectations. References: https://trailhead.salesforce.com/en/content/learn/modules/user-acceptance-testing-video/learn-about-user-acceptance-testing