The Scrum Master is responsible for promoting and supporting Scrum as defined in the Scrum Guide.
Scrum Masters do this by helping everyone understand Scrum theory, practices, rules, and values.

Explanation
B: Productivity, in terms of lines of code or story points, will probably suffer during the transition, although even then delivery of business value is still likely to increase. This is a realistic expectation, as the teams will need to learn new skills, collaborate more effectively, and adapt to changing requirements. However, the benefit of feature teams is that they can deliver working software that provides value to thecustomers and stakeholders every Sprint, rather than waiting for the integration of different components.
C: Feature teams will require time to become productive as people from the different layers and components become accustomed to working and delivering unified functionality together, as one Scrum Team. This is a natural consequence of changing the team structure and culture, as the teams will need to overcome some challenges and conflicts, build trust and rapport, and establish a shared vision and goal. The Scrum Master can help the teams with this transition by facilitating communication, coaching, and mentoring.
References:
Professional Scrum Master II Course, page 17: "Feature teams are cross-functional teams that can deliver end-to-end functionality for a product. They have all the skills and competencies needed to work on multiple layers of the system." Professional Scrum Master II Course, page 18: "Component teams are teams that specialize in one layer or component of the system. They often depend on other teams to deliver a complete functionality for a product." Professional Scrum Master II Course, page 19: "Feature teams have many advantages over component teams, such as faster feedback, higher quality, lower complexity, and more customer value."

The Development Team owns the Definition of "Done" and is accountable for the quality of the Product Backlog items.

Coach the Product Owner that achieving the Sprint Goal is the Scrum Team's objective. A forecast is only used to initially estimate the effort required to reach the Sprint Goal. This is true because the Scrum Master's role is to help the Product Owner understand the Scrum framework and its principles, such as empiricism, self-organization, and collaboration. The Scrum Master should also remind the Product Owner that the forecast is not a commitment, but a plan that can be adjusted based on the Sprint progress and feedback. The Scrum Master should focus on the value and outcome of the Sprint, rather than the output and scope.