C. To produce regular highlight reports for the project board The ‘controlling a stage’ process involves monitoring and managing the work within a stage to ensure that it remains on track. This includes producing regular highlight reports that provide the Project Board with updates on progress, issues, and risks. These reports help the Project Board make informed decisions and ensure that the stage objectives are being met effectively.

Here is the description straight out of the prince2 7th addition manual. I think it should be C

Purpose: The purpose of the process of controlling a stage is to assign work, monitor such work, handle issues, report progress to the project board, and take corrective actions to ensure that the stage remains within the tolerances set by the project board