The committee that sits together to assess your research proposal, sits with an objective and the objective is to focus on your research question. They want to understand that the question that you have given in your proposal is good/strong/substantive enough to allow you to give a significant contribution to an existing body of literature. As a research scholar and standing in defence of your thesis, it is your job to convince the panel about the relevance and importance of your research question.
Your research proposal is good when you have clearly presented the research question in your proposal itself, however the scholars who are in the advanced stage of their research would agree that as your progress and proceed with your research, your research question upgrades and expands onto another level from where you began in the initial stage. When you meet the review committee periodically, it gives you and them to reflect onto the question and its relevance in the changed circumstances and whether your research is in the correct line and well synced with answering the question in the capacity you had proposed to do. It is a good idea that every time you prepare the report of the progress of your work you present not just your research question but a justified rationale for it that emphasises its rationality and your attempt in answering it becomes clearer.
When you provide a rationale to the research question, you have largely convinced the committee in the beginning itself about its appropriateness in your PhD. You must explain the rationale why is it important in light of what all has already been researched in that area and how the answer that you give to the question helps to fill that gap that exists in the existing literature. The rational can be made more holistic and impressive by giving the practical implications of the question, justifies its topicality and relevance in the current scenario and also discusses how the research would open the doors and create the scope for further and more advanced research. You should be able to argue your case so convincingly that the panel has to agree to the claims you have put up. When you highlight how it contributes to the existing body of literature before you are asked to think about it, it is surely convincing that you know your research well and you have caught the bull by its horns so your research is surely on its track.