In an agile project, the product owner is responsible for defining and prioritizing the product backlog, and the team is responsible for delivering the product increment based on the sprint backlog. If the product owner identifies that some key features are missing from the product increment, the project manager should collaborate with the product owner and the team to plan the next sprint and adjust the backlog accordingly.
Extending the sprint timeline (A) would violate the time-boxed nature of agile sprints and disrupt the rhythm and cadence of the project. Informing the product owner that the issue needs to be escalated (B) would not solve the problem, but rather create unnecessary conflict and bureaucracy. Initiating the change request process (D) would not be appropriate for an agile project, as changes are expected and welcomed throughout the project lifecycle, and do not require formal approval or documentation. References: PMBOK Guide, 7th edition, page 91; Agile Practice Guide, page 14.