Explanation
According to the Agile Practice Guide, the product owner is responsible for managing the product backlog, which includes adding, removing, or updating product backlog items as needed. However, the product owner should not change the scope of the sprint once it has started, as this would disrupt the team's commitment and velocity. Therefore, the best option for the project manager is to create a new product backlog item for the next sprint planning, where the team can review and estimate it. This way, the change request is properly documented and prioritized, and the team can deliver the agreed-upon functionality for the current sprint.
References:
1: Agile Practice Guide, page 35
2: Agile Practice Guide, page 37