How often should the Product Backlog be refined?

Correct answer: Continuously throughout the Sprint

Option D is correct because the Scrum Guide states that Product Backlog refinement is an ongoing activity that happens continuously throughout the Sprint, not as a single formal event; the Development Team and Product Owner collaborate regularly to decompose and clarify backlog items so that upcoming items are ready for Sprint Planning. Option A is incorrect because limiting refinement only to before Sprint Planning creates a bottleneck and does not allow adequate time for thorough discussion and decomposition of complex items. Option B is incorrect because refining only at the end of each Sprint ties the activity to a single point in time rather than the continuous cadence the Scrum Guide prescribes. Option C is incorrect because quarterly refinement is far too infrequent for an iterative framework that operates in short Sprints and continuously adapts its backlog.