Final answer:
The Product Owner is required to be present at the Sprint Retrospective for a comprehensive assessment and improvement process.
Step-by-step explanation:
The correct answer is option C, their attendance is mandatory. The Sprint Retrospective is an opportunity for the Scrum Team to assess and improve itself. The Product Owner plays a crucial role in the Scrum framework and is an integral part of the Scrum Team. Therefore, their presence in the Sprint Retrospective is necessary to ensure a comprehensive assessment and improvement process. During the Retrospective, the Scrum Team reflects on the most recent Sprint and identifies areas for improvement.