The correct option is The subtask will be treated as a scope change and it will be shown in the burndown chart.

Because the Scrum board has Remaining estimate and Time spent enabled, adding a subtask to an issue that is already in the active sprint increases the sprint's total remaining estimate when the subtask has an estimate. This increase is treated as a sprint scope change and the burndown chart will reflect the change in remaining work so the new subtask appears on the chart.

The subtask will be counted as a scope change but it will not update the burndown chart is incorrect because when remaining estimates are tracked the burndown chart updates to show the added remaining work. A scope change that affects remaining estimates will be visible on the chart.

The subtask will not change sprint scope but it will appear on the burndown chart is incorrect because a new subtask added to an issue in the active sprint does change the sprint scope if it carries remaining estimate. It cannot appear on the burndown without affecting the tracked remaining work.

The subtask will not alter sprint scope and it will not be reflected in the burndown chart is incorrect because adding a subtask with an estimate during an active sprint alters the sprint scope and the burndown chart will reflect that added remaining work.

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