In Scrum, the product owner is the sole person responsible for managing the product backlog. When a customer requests new features mid-sprint, it is inappropriate to add them directly to the sprint. The correct procedure is to have the Scrum Master direct the customer to the product owner, who will evaluate the request and decide whether to include it in the product backlog for future prioritization.
According to the Agile Practice Guide and the Scrum Guide, only the product owner has the authority to accept or reject items in the product backlog. The Scrum Master facilitates communication and protects the team from scope creep during the sprint.
Option A (Definition of Done) is useful for clarifying quality, not for managing scope changes.
Option C (Acceptance criteria) applies to existing backlog items, not new feature requests.
Option D (Backlog prioritization) is the product owner's job and only happens after the sprint, not mid-sprint.
References:
Agile Practice Guide, Chapter 3: Roles and Responsibilities
The Scrum Guide 2020: Product Owner and Scrum Master Roles
PMBOK Guide - Seventh Edition, Agile Tailoring Considerations