According to the Scrum Guide, the Developers are the people in the Scrum Team who are committed to creating any aspect of a usable Increment each Sprint. They are accountable for creating a plan for the Sprint, the Sprint Backlog, and for adhering to the Definition of Done. The Product Owner and the Scrum Master are not accountable for these activities, but they may support the Developers as needed. The Scrum Team as a whole is accountable for delivering a valuable, useful, and potentially releasable Increment each Sprint, but the Developers have the specific accountability for planning and building it. Reference := Scrum Guide, Understanding and Applying the Scrum Framework, Managing Products with Agility

The smallest product Increment that is valuable enough to release is one that delivers a single new or improved outcome for the customers or users of the product. This is because the outcome is the ultimate measure of value, not the output or the features1. An outcome is a change in behavior, attitude, or situation that results from using the product2. An outcome can be achieved by delivering a single feature, fixing a defect, or improving an existing functionality, as long as it meets the Definition of Done and the Sprint Goal3.
Therefore, the best answer is A.
The other options are not correct because:
* B. Adds a new feature. This is not a sufficient condition, as a new feature may not deliver a valuable outcome if it is not aligned with the product vision, the customer needs, or the market conditions4.
* C. Fixes at least one defect. This is not a necessary condition, as a defect may not affect the value of the product if it is minor, rare, or irrelevant to the customer5.
* D. Delivers all "must do" Product Backlog items. This is not a realistic condition, as the Product Backlog is a dynamic and emergent artifact that changes based on feedback, learning, and discovery. There is no fixed set of "must do" items that can guarantee the delivery of value. References: 1: Outcome vs. Output, 2: What is an Outcome?, 3: The Definition of Done, 4: Feature vs. Outcome, 5:
Defect vs. Value, : The Product Backlog