The value of a product is not determined by how many features it has, how much it costs, or how fast it is delivered, but by how well it meets the needs and expectations of the customers and stakeholders. Therefore, the best measure of value is how often and how effectively the customers use the product to achieve their goals and solve their problems. This can be assessed by using metrics such as customer satisfaction, retention, engagement, loyalty, referrals, revenue, or any other indicators that reflect the desired outcomes and benefits of the product.
The other options are not the best measures of value, because they either focus on the output rather than the outcome, or they do not reflect the customer perspective. The number of "must-do" Product Backlog items delivered in a release may indicate the scope or the quality of the product, but not necessarily the value. The average cost of the product release may indicate the efficiency or the profitability of the product, but not necessarily the value. The on-schedule performance of the Scrum Team may indicate the predictability or the agility of the product development process, but not necessarily the value. References := Professional Scrum Product Owner II Certification, Managing Products with Agility, Evidence-Based Management