Explanation
None of the options given are accurate descriptions of what an effective Product Owner must do. According to the PSPO II resources, an effective Product Owner must12345:
* Develop and communicate a clear and compelling Product Goal that aligns with the organization's vision and strategy.
* Manage the Product Backlog by refining, ordering, and prioritizing items that deliver value to customers and users.
* Collaborate with stakeholders and the Scrum Team to understand their needs, expectations, and feedback.
* Empower the Scrum Team to make decisions and self-organize around the Sprint Goal and the Product Backlog items.
* Experiment and validate assumptions using empirical evidence and data.
* Optimize the value of the product and the work of the Scrum Team.
* Continuously inspect and adapt the product and the process based on the outcomes and feedback.
References:
* 1: The Scrum Guide
* 2: Product Owner Learning Path
* 3: Product Owner Competencies
* 4: Evidence-Based Management
* 5: Product Strategy

A burndown chart is a graphical tool that shows the amount of work remaining versus time. It is often used to track the progress of a Sprint or a release. The work remaining is usually measured in terms of effort, such as hours or story points, and it is plotted on the vertical axis. The time is measured in terms of days or weeks, and it is plotted on the horizontal axis. The burndown chart starts with the total amount of work at the beginning of the Sprint or release, and it ideally ends with zero work at the end. The slope of the burndown chart indicates the rate of progress, or the velocity, of the Scrum Team12.
The other options are not what burndown charts track. Accumulated cost, accumulated business value, and individual worker productivity are not relevant or useful metrics for Scrum Teams, as they do not reflect the value or the quality of the product. Scrum Teams focus on delivering potentially releasable increments of the product that meet the Definition of Done, and they use empirical feedback to inspect and adapt their work13. References: 1: Understanding and Applying the Scrum Framework 2: Burndown Chart 3: Managing Products with Agility