The correct option is Post function.

Post function runs as part of a workflow transition and executes automatically when an issue moves between statuses. You can add a post function to set or change the Assignee field during the transition so the issue is reassigned as the transition completes. This is the standard Jira workflow mechanism to make programmatic changes tied to a specific transition.

Automation rule is incorrect because automation executes separate rules that are not configured inside the workflow transition itself. Automation can change assignees based on events but it is not the built in transition step used to ensure the assignee is set at the moment the workflow transition runs.

Transition screen is incorrect because a transition screen only collects input from the user during a transition and does not by itself set values automatically. A screen can allow the user to set the Assignee but it will not programmatically reassign the issue unless combined with a post function or user input.

Project component is incorrect because components are a way to group work and optionally designate component leads but they do not control reassignment during specific workflow transitions. Components do not act as transition actions.

Field configuration is incorrect because field configurations control field behavior such as required or hidden status and validation rules. They do not perform actions that set or change field values during a transition.

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