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.
References:
1: Project Management Institute. (2023). A Guide to the Project Management Body of Knowledge (PMBOK Guide) - Seventh Edition. Newtown Square, PA: Author.

Organizing more frequent sprint reviews with a broader audience is a key practice in Agile methodologies to ensure that the increments meet the end users' quality requirements. This approach allows for continuous feedback and collaboration, which are essential for Agile teams. It enables the team to present the increment to stakeholders, including end users, and receive immediate feedback that can be incorporated into the next sprint. This aligns with Agile principles of customer collaboration and responding to change123.
References:
* Agile QA Process: Principles, Steps, and Best Practices1
* Best Practices for an Agile QA Process2
* How to Ensure Quality in Agile: Best Practices and Tools3

Diagramming Techniques (including Ishikawa diagrams and Flowcharts) are a tool of the Risk Identification process. You use them to find the root cause of defects in Quality Management processes but they can also be useful in finding the risks that can lead to trouble in Risk Management.