According to the PMBOK Guide, a business case is a document that provides the justification for undertaking a project or an additional scope, based on the estimated costs, benefits, and risks. A project manager should develop a business case and submit it to the client when they realize that some minor changes could deliver significant benefits to the client early. This will help to communicate the value proposition of the changes, and to seek the client's approval and support. Developing a business case and submitting it to the client is the best approach that the project manager should take in this situation. Concentrating on the current project milestones (A) may not be optimal or beneficial, as it may miss the opportunity to enhance the client satisfaction and project outcomes. Updating the project management plan for this new work (B) may not be feasible or ethical, as it may violate the project scope, schedule, and budget baselines, and the agreed-upon project objectives and deliverables. Making all the changes and then showing the client the results (D) may not be appropriate or acceptable, as it may create scope creep, increase project risks, and damage the trust and relationship with the client. References: PMBOK Guide, 6th edition, pages 33, 75, 129.