Hi Gerallt. Agile Project Chartering serves the purpose of creating a shared vision among all stakeholders. As Mike Griffiths mentions in his book, PMI-ACP Exam Prep, "It describes the project's purpose, composition, and approach, and it provides the authorization from the sponsor for the project to proceed."
Although not directly addressed in the book, the Agile Project Charter could be owned by the Product Owner, or Product Management, while input to craft the document could be provided various stakeholders. As a Scrum Master, I produced the Agile Project Charter at the beginning of projects with assistance of the Product Owner, while other Projects the Product Owner produced it with my assistance.
For the purposes of the test, I'd stick to understanding the intent of the Agile Project Charter and the "agile mindset". It's highly unlikely that you'll see a simple question like "Who is responsible for..." or "Who produces the Agile Project Charter?" The test will likely give you a situational question asking you to pick the best way to establish a common understanding given the circumstances.
Hope that helps.
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