When does a Sprint conclude?

According to the Scrum Guide, a Sprint concludes when the Sprint Retrospective is complete. This is the last event of the Sprint cycle, where the Scrum Team inspects itself and creates a plan for improvements. The other options are not valid indicators of the conclusion of a Sprint, as they are either irrelevant (such as the Product Owner deciding enough has been delivered) or incomplete (such as all tasks or Product Backlog items being done).