The correct answer is status was not Completed.

status was not Completed uses JQL historical searching to return issues that have never been in the Completed status. The operator "was not" looks at the change history of the status field and excludes any issue that at any point had the Completed status, so only issues that have never been completed are returned.

status != Completed is incorrect because it only checks the current status of issues. That query will list issues that are not currently Completed but it will include issues that were Completed in the past and then moved out of that status.

resolution is EMPTY is incorrect because resolution is a different field than status. An empty resolution finds unresolved issues but it does not guarantee the issue was never in the Completed status since resolution can be changed independently of status.

status was in ("Backlog","Active") is incorrect because it finds issues that were ever in Backlog or Active. That query does not exclude issues that were also in Completed at some point, so it does not satisfy the requirement.

status changed TO Completed is incorrect because it finds issues that have been moved into Completed at least once. That is the opposite of what the question asks for, since those issues have been in Completed.

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