According to the PMBOK Guide, project managers should apply adaptive planning and agile practices to respond to changes and uncertainties in complex projects. Adaptive planning involves frequent reviews and adjustments of the project scope, schedule, cost, and quality based on the feedback and learning from the project execution. Agile practices emphasize delivering value to the customer by prioritizing the most important features and requirements and delivering them in increments1.

In this scenario, the project manager faces a situation where one feature cannot be fixed before the go-live date, which impacts the project schedule and quality. The project manager should apply adaptive planning and agile practices to determine the best course of action. The project manager should evaluate the value and importance of the feature to the customer and the project objectives. If the feature is not critical or essential, the project manager should consider delivering the project without the feature and fixing the defects later, after the go-live milestone. This way, the project can still deliver value to the customer and meet the expectations of the stakeholders. The project manager should communicate this decision to the customer and the stakeholders and obtain their agreement and feedback1.

The other options are not as suitable as option D. Option A involves updating the risk management plan and including additional mitigation actions against the feature defects. However, this option does not address the impact on the go-live milestone or the customer value. It also assumes that the feature defects can be mitigated, which contradicts the premise of the question. Option B involves assessing the relevance of the feature and submitting a change request to descope it. However, this option does not consider the customer value or the feedback from the customer and the stakeholders. It also implies that the feature is not important or valuable, which may not be the case. Option C involves improving the control quality process by implementing additional testing to minimize the defects before the go-live milestone. However, this option does not acknowledge the fact that there is no way to resolve the defects before the go-live date, as stated in the question. It also does not consider the impact on the project schedule and cost, or the customer value1.