The Product Owner's core accountability is maximizing value through the Product Backlog. The Scrum Guide states: 'The Product Owner is accountable for maximizing the value of the product resulting from the work of the Scrum Team.' Option B encapsulates this fully.

A: The Developers deliver the Increment; the Product Owner doesn't ensure it.

C: Refinement is part of management, but not the sole accountability.

D: Writing User Stories is a technique, not the overarching duty.

Exact Extract from Scrum Guide: 'The Product Owner is accountable for effective Product Backlog management, which includes: Developing and explicitly communicating the Product Goal; Creating and clearly communicating Product Backlog items; Ordering Product Backlog items; and, Ensuring that the Product Backlog is transparent, visible and understood.' (Section: 'The Product Owner')

Thus, B is correct.