Automation rule and Workflow post functions are correct because they can be used to automatically clear the Fix Status field when an agent reopens a ticket.

Automation rule can trigger on the issue reopened event and then run an action to update or clear the Fix Status field. Automation rules are efficient for handling repeated events across issues and they do not require modifying the workflow for every project where the behavior is needed.

Workflow post functions run as part of a transition such as a reopen transition and can update fields during that transition. Use a post function when you want the field change to be tied directly to the workflow step so the update happens reliably during the transition.

Workflow conditions are checks that decide whether a transition can proceed and they do not perform field updates, so they cannot be used to clear the Fix Status field on reopen.

ScriptRunner validator runs before a transition to validate data and possibly block the transition. A validator is not intended to change field values during the transition, so it is not the right mechanism for clearing a field.

Bulk change operation is a manual operation to modify many issues at once and it does not run automatically when an agent reopens a single ticket, so it does not meet the automation requirement.

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