The question here asks about the process that fetches the initial list of key stakeholders.
Choice A - Develop Project Charter - Correct - Project Charter, the main output of Develop Project Charter process contains key stakeholder list and high level classification of roles and responsibilities of everyone involved in the project.
Choice B - Stakeholder Register - Incorrect - It is an example of a Project Document that records all Stakeholder information.
Choice C - Business Case - Incorrect - It is a Business Document which is input for the Develop Project Charter process. Business case justifies why the project was undertaken.
Choice D - Stakeholder Engagement plan - Incorrect - It is an output for the Plan Stakeholder Engagement Process. It provides actionable plan to interact effectively with the stakeholders.
(PMBOK Guide) – Sixth Edition, Project Management Institute Inc., 2017, Page(s) – 81, 509.