Explanation/Reference:

A Product Backlog item is a description of a feature or functionality that adds value to the product.
It is selected by the Scrum Team during the Sprint Planning, based on the Product Owner's proposal and the Developers' forecast. It is expected that the Product Backlog item will be completed by the end of the Sprint, meaning that it meets the Definition of Done, which is a formal description of the state of the Increment when it meets the quality standards required for the product. However, if a Product Backlog item is not completed by the end of the Sprint, meaning that it does not meet the Definition of Done, then:
The item is not included in the Increment for this Sprint (B), which is a valid option as it ensures that only "Done" work is delivered to the customer and stakeholders. The Increment is a concrete and usable outcome of a Sprint that provides value and feedback. It should not contain any undone or partially done work that may compromise its quality, usability, or value. It is put on the Product Backlog for the Product Owner to decide what to do with it (D), which is another valid option as it respects the authority and accountability of the Product Owner to manage the Product Backlog, which is an ordered list of what is needed in the product. The Product Owner can decide whether to re-prioritize, re-estimate, refine, or remove the incomplete Product Backlog item based on new insights and stakeholder needs.

This is a valid concern, as the Daily Scrum is an event where the Developers can inspect the progress toward the Sprint Goal and adapt the Sprint Backlog accordingly. If the Daily Scrum is held less frequently, the Developers may miss some opportunities to adjust their plan and collaborate effectively.
This is a valid concern, as the Daily Scrum is an event where the Developers can share their work status, challenges, and dependencies with each other. If the Daily Scrum is held less frequently, the Developers may have less visibility and awareness of what is going on in the Sprint, and may face more surprises or risks.
This is a valid concern, as the Daily Scrum is an event where the Developers can identify and communicate any impediments or issues that hinder their work. If the Daily Scrum is held less frequently, the Developers may have to wait longer to get help or support from their peers or the Scrum Master, and may lose some momentum or efficiency.