According to the PMBOKGuide 7th Edition, one of the principles of project management is to "focus on value". This means that the project manager should deliver products, services, or results that meet the needs and expectations of the customer and other stakeholders. One of the ways to achieve this is to use an adaptive or agile approach, which allows for frequent feedback and collaboration with the customer. Working with the product owner to define the minimum viable product(s) (MVPs) is an example of an agile technique that helps to deliver value early and often, and to validate the customer's requirements. MVPs are versions of the product that have enough features to satisfy the customer's needs and provide feedback for future development.
Therefore, this option is the best approach to remove the impediment of the customer's unclear requirements.
The other options are not as effective because they do not address the root cause of the problem, which is the lack of customer involvement and validation. Following a predictive approach may lead to delivering a product that does not meet the customer's needs or expectations, and may result in rework or waste.
Reinforcing the negotiation and soft skills of the team may help to improve the communication and relationship with the customer, but it does not ensure that the product will deliver value. Developing and monitoring a plan with due dates may help to manage the schedule, but it does not guarantee that the product will satisfy the customer's requirements. References: : PMBOKGuide 7th Edition, PMI, 2021, p. 9 : Agile Practice Guide, PMI, 2017, pp. 25-26