Hi Dalia.
For PMP type questions, the correct answer amongst the choices is usually the "correct" and "best" option. This is because in most cases ALL the options are CORRECT and the aim is for you to pick the BEST among the pack.
Now to your question; the answer you gave is "stakeholder engagement plan" which though correct, is not the best answer among the pack. This is because the "stakeholder engagement plan" is a broad plan encompassing all actions relating to the stakeholders. Within the stakeholder engagement plan, we have stakeholder engagement assessment matrix, stakeholder register and stakeholder analysis. My choice among the 3 would be stakeholder analysis.
This is because the stakeholder engagement assessment matrix is a data representation technique and the question is asking "What should the project manager use to manage each stakeholders influence on the project?" The other option of stakeholder register simply lists down the stakeholders in a project but it is not used to "manage each stakeholders influence on the project." I hope that helps.