Doesn't seem right. All other quizzes made me clearly understand the Product Owner has nothing to do with the Sprint Retrospective, in opposition with the Sprint Review which he leads along with the key users. Sprint Retrospective is to allow the Developer Team to improve its auto-organization.
It is mandatory. The Sprint Retrospective is an opportunity for the Scrum Team to assess its performance and improve itself.
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.