During a Sprint Retrospective, for what is the Scrum Master responsible?

According to the Scrum Guide1, the purpose of the Sprint Retrospective is to plan ways to increase quality and effectiveness. The Scrum Team inspects how the last Sprint went with regards to individuals, interactions, processes, tools, and their Definition of Done. Inspected elements often vary with domain, practices, and people involved. The Scrum Team identifies the most helpful changes to improve its effectiveness. The most impactful improvements are addressed as soon as possible. They may even be added to the Sprint Backlog for the next Sprint. The Sprint Retrospective is an opportunity for the Scrum Team to inspect itself and create a plan for improvements to be enacted during the next Sprint. The Scrum Master is part of the Scrum Team and should participate as a Scrum team member and facilitate as requested or needed.

References: Scrum Guide