The correct answer is Pipeline trigger. It is the element in Azure Data Factory that causes a pipeline to run.

Triggers are Azure Data Factory resources that start pipelines based on schedules, events, or tumbling windows and they can also be started manually. When a trigger fires the service creates a pipeline run and the pipeline activities execute within that run.

Pipeline parameter is an input value that you pass to a pipeline or to activities and it does not by itself start a pipeline. Parameters make pipelines reusable and configurable at runtime but they are not initiators.

Control flow element refers to constructs and activities inside a pipeline such as If Condition or ForEach that manage the execution path during a run. These control flow elements do not launch a pipeline from an idle state.

Pipeline activity is an action performed inside a pipeline such as copying data or executing a stored procedure and activities execute as part of a pipeline run. Activities do not independently cause a pipeline to start.

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