The Sprint Backlog is a living artifact that evolves and changes as more is learned or discovered.

These three pillars uphold every implementation of the empirical process control. Without them, Scrum cannot be implemented as intended.

Explanation
The best way to determine if the Product Owner is interacting with the Developers enough throughout a Sprint is to observe whether both parties are satisfied with the Increment. The Increment is a concrete stepping stone toward the Product Goal1. It must be usable and meet the Definition of Done1. The Product Owner is accountable for maximizing the value of the product resulting from the work of the Developers1. The Developers are accountable for creating a valuable, useful, and potentially releasable Increment every Sprint1.
Therefore, if both the Product Owner and the Developers are happy with the quality and value of the Increment, it means that they have communicated and collaborated effectively throughout the Sprint.
Some reasons why the other options are not correct are:
A: The Developers should determine the percentage of time the Product Owner is required to be present and monitor whether the Product Owner's average presence is around this figure. This option is not correct because it implies that there is a fixed or optimal amount of time that the Product Owner should spend with the Developers, which may not be true. The Product Owner's interaction with the Developers may vary depending on the context, complexity, and uncertainty of the product and the Sprint. The Scrum Guide does not prescribe any specific time orfrequency for the Product Owner's involvement with the Developers1. The Product Owner should be available to provide guidance and feedback to the Developers as needed, but not interfere with their self-organization and autonomy1.
B: Check the Daily Scrum reports to see whether the Product Owner was present at the majority of the Daily Scrum meetings. This option is not correct because it assumes that the Daily Scrum is a meeting where the Product Owner should be present, which is not true. The Daily Scrum is an event for and by the Developers12, not for reporting or accountability to anyone else. The Product Owner may attend as an observer or a collaborator, but only if invited by the Developers12. The presence or absence of the Product Owner at the Daily Scrum does not indicate how much they interact with the Developers throughout the Sprint.
C: Observe whether the Developers need to ask the Product Owner questions at times other than during the Daily Scrum. This option is not correct because it implies that asking questions is a sign of insufficient interaction, which may not be true. Asking questions is a natural and healthy part of communication and collaboration between the Product Owner and the Developers. It shows that they are engaged, curious, and willing to learn from each other. It also helps them to clarify requirements, expectations, and feedback, and to resolve any ambiguities or conflicts. Asking questions does not necessarily mean that there is a lack of interaction, but rather that there is a need for more information or clarification.
References:
What is an Increment? | Scrum.org
What Is a Daily Standup?| A Guide to Running Effective Standup Meetings | Atlassian

The Definition of Done helps the Scrum Team have a shared understanding of what it means for work to be complete, to ensure transparency. The Definition of Done is used to assess when work is complete on the product Increment. It also ensures that the work that is done is in usable condition and meets the Development Teams quality standards for future sustainability.