When a project manager identifies a risk with a high probability of occurring, the appropriate response is to monitor the risk closely. This involves asking the team to keep an eye on the risk and provide alerts if it becomes an issue. This approach is aligned with the proactive risk management practices advocated by PMI, where risks are continuously monitored and managed throughout the project lifecycle. It allows the project team to respond quickly to changes and implement risk response plans as necessary.
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The PMBOK Guide - Sixth Edition provides guidelines on risk management processes which include monitoring risks. It emphasizes the importance of keeping track of identified risks, monitoring residual risks, and reviewing the execution of risk responses while evaluating their effectiveness throughout the project lifecycle1.
The Practice Standard for Project Risk Management by PMI also supports this approach, detailing how monitoring and controlling risks is a critical part of the risk management process2.
Additionally, the PMP Exam Content Outline from PMI outlines the tasks associated with monitoring risks, which includes implementing risk response plans, tracking identified risks, and evaluating risk process effectiveness over the project's duration3.

According to the PMBOK Guide, when using an agile approach, the project manager needs to deliver value early and often by creating and testing increments of the product, service, or result that are potentially releasable1. A minimum viable product (MVP) is a version of the product that has enough features to satisfy the initial needs of the customers and stakeholders, and to provide feedback for future development2. By developing an MVP for assessment from a select customer group, the project manager can address the issue of the product owner's uncertainty about the business value of each feature, as well as validate the product vision, test the assumptions, and reduce the risks2. The other options are not appropriate, as they do not reflect the agile mindset and principles. Assessing the product roadmap, commissioning a business analyst, or completing a WBS are all possible actions that could be taken in a predictive or waterfall approach, where the product features are defined and fixed upfront. However, in an agile approach, the product features are emergent and adaptive, and the project manager should collaborate with the customer and the team to deliver the best possible product3. References:
* 1: PMBOK Guide, 6th edition, page 18
* 2: PMBOK Guide, 6th edition, page 158
* 3: PMBOK Guide, 6th edition, page 156

The project manager should prioritize removing barriers that impede team performance and autonomy. This action aligns with the principles of agile project management, which emphasize empowering team members to make decisions and take ownership of their work12. By addressing the internal procedures that are hindering progress, the project manager can facilitate a more efficient and autonomous environment, allowing the team to focus on delivering value more effectively3.
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PMI's resources on team building and the importance of removing barriers to enhance team performance1.
Literature on expected behaviors for project team performance, which includes empowering team members and promoting autonomy2.
Agile methodologies that advocate for team empowerment and the removal of impediments to improve project outcomes3.

These two actions can help mitigate the risk of staff turnover during the course of a large multi-year project, because they can increase the satisfaction, motivation, and loyalty of the project team members. A recognition program can show appreciation and gratitude for the team members' efforts and achievements, and provide positive reinforcement for their desired behavior. Training, mentoring and coaching can improve the skills and performance of the team members, and provide them with opportunities for learning and growth. These actions can also foster a positive and collaborative team culture, and enhance the trust and communication between the project manager and the team members. The other options are not effective actions to mitigate the risk of staff turnover, because they may not address the root causes or consequences of the risk, or they may create more problems or risks. Increasing the trained staff to create back-up for resources may not be feasible or cost-effective, and it may create redundancy, confusion, or conflict among the team members. Meeting with management to discuss dividing the project into smaller projects may not be possible or desirable, and it may affect the scope, quality, or benefits of the project. Convincing senior management to hire third-party staff may not be beneficial or reliable, and it may increase the complexity and risk of the project. References: Positive Reinforcement | PMI - Project Management Institute, [The Mindful Project Manager's Guide To Team Motivation: 11 ... - Planio].