According to the PMBOK Guide, one of the tools and techniques for collecting requirements is prototypes1.
This involves developing a preliminary version or model of the product, service, or result that is expected from the project, and using it to elicit feedback from the stakeholders. By discussing the project scope statement with the key stakeholder and building prototypes, the project manager can improve the stakeholder's engagement, as well as clarify and validate the requirements, reduce changes, and enhance satisfaction1. The other options are not appropriate, as they do not address the issue of the stakeholder's frequent requests for changes to the project scope. Discussing the WBS, updating the project backlog, or updating the project charter are all possible actions that could be taken after collecting the requirements, depending on the project methodology and life cycle. References:
* 1: PMBOK Guide, 6th edition, page 149