An organization cannot start an important project because scope definition is blocking the start. Most scope items are defined and agreed, but a few are complex and hard to define now. What should the project manager do?

The correct answer is to propose a hybrid approach. This allows the project to start immediately with the well-defined scope items using a predictive (plan-driven) approach, while the complex, hard-to-define items can be tackled iteratively and adaptively using an agile approach. This maximizes value delivery and addresses uncertainty effectively. Maintaining stakeholder momentum until a fully detailed scope is achieved is impractical and delays the project unnecessarily, potentially losing momentum. Splitting the project into two is an unnecessary complication and doesn't address the core issue of defining complex scope. Updating the risk register and escalating to the PMO is a reactive measure; while important for risk management, it doesn't proactively solve the scope definition problem or allow the project to begin.