The correct answer is status was not Completed.

The JQL status was not Completed uses the history operator was not to find issues whose status field has never been the value Completed at any point in their change history. This query therefore returns issues that have never been in the Completed state, which matches the question requirement.

resolution is EMPTY is incorrect because it checks whether the current resolution field is empty and it does not inspect the status history. An issue can have no resolution and still have been in Completed, or it can never have been Completed and still have a resolution set.

status was in ("Backlog", "In Development") is incorrect because it finds issues that were at some time in Backlog or In Development but it does not exclude issues that were also in Completed at some point. It does not answer whether Completed was ever used.

status != Completed is incorrect because it only filters the current status and will return issues that are not currently Completed. It will return issues that were Completed in the past but have since moved to another status.

resolution != null is incorrect because it tests the resolution field and not the status history. This does not guarantee anything about whether the issue was ever in the Completed status.

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