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

Explanation
= As a Product Owner, you are accountable for maximizing the value of the product and the work of the Developers. To do this, you need to collaborate with the Developers andthe Scrum Master to ensure that the Product Backlog is clear, ordered, and refined, and that the Sprint Goal and the Sprint Backlog are aligned with the product vision and strategy. You also need to provide feedback and guidance to the Developers throughout the Sprint, and to inspect and adapt the product based on the Sprint Review and the stakeholders' input.
One of the challenges that the Developers may face is to forecast the amount of work that they can complete within a Sprint, and to deliver a potentially releasable Increment that meets the Definition of Done. This requires the Developers to have the skills and the tools to estimate the complexity and the effort of the Product Backlog items, to plan and manage their work effectively, and to adhere to the quality standards and the technical practices that enable them to build a valuable and usable product increment.
To help the Developers improve their ability to deliver a done Increment, the Product Owner can take the following steps:
* Ask the Scrum Master to help the Developers learn techniques for improving their ability to forecast work. The Scrum Master is accountable for establishing an environment where the Developers can work effectively, and for coaching the Developers on self-organizing and cross-functionality. The Scrum Master can also help the Developers to apply empirical process control, to use various estimation methods, to monitor and visualize their progress, and to inspect and adapt their plan based on the Sprint Backlog and the Sprint Burndown Chart.
* The Product Owner can spend more time with the Developers. The Product Owner can support the Developers by clarifying the Product Backlog items, providing the acceptance criteria, explaining the customer needs and the business value, and answering any questions that the Developers may have. The Product Owner can also participate in the Sprint Planning, the Daily Scrum, and the Sprint Retrospective, to share their insights, expectations, and feedback, and to collaborate with the Developers on creating and refining the Sprint Goal and the Sprint Backlog.
The other options are not the best steps to take, because they either do not address the root cause of the problem, or they may have negative consequences. Ensuring that all Developers are top performers may not be realistic or feasible, and it may also create a culture of blame or competition, rather than collaboration and learning. Adding more people to the team may not necessarily increase the productivity or the quality of the work, and it may also introduce communication and coordination challenges, as well as additional costs and risks. References := Scrum Guide, Understanding and Applying the Scrum Framework, Managing Products with Agility

According to the Scrum Guide, the Scrum Team consists of one Scrum Master, one Product Owner, and Developers. The entire Scrum Team is accountable for creating a valuable, useful Increment every Sprint1. The Increment is a concrete step toward achieving the Product Goal, and it must meet the Definition of Done and be usable by the stakeholders1. The Product Owner is accountable for maximizing the value of the product and the work of the Developers1. The Scrum Master is accountable for establishing Scrum as defined in the Scrum Guide, helping everyone understand Scrum theory and practice, and removing impediments to the Scrum Team's progress1. The Developers are accountable for creating any aspect of a usable Increment each Sprint1. All three roles must collaborate and coordinate their efforts to deliver the best possible product2.

According to the Professional Scrum Product Owner™ II certification guide1, the Product Owner is accountable for maximizing the value of the product resulting from the work of the Scrum Team. This means that the Product Owner is responsible for defining, ordering, and validating what the Scrum Team works on, and ensuring that the product delivers value to the customers, users, and the organization. The other options are not the most important thing a Product Owner can do, because they are either too narrow (A), too unrealistic (B), or too prescriptive ©. Reference: 1: Professional Scrum Product Owner™ II Certification | Scrum.org