I am asked to discuss the underpinning of my methodology. ... my methodology include structured questionnaire... so what is my supervisor looking for ?
Your supervisor is looking for the theoretical Framework of the methodology you are using. So synthesise the theories underpinning your methodologies, where it has been used and what u intend to add to improve it limitations if any.
Every methodology has to be based on strong theoretical foundation. From the literature review you have to identify the research problem that leads to identification of research objective. With strong literature review you have to justify how you are going to address the research objective. What are the factors that needs to be identified or evaluated for your research purpose also needs to be justified through review of literature. You also have to justify how your proposed research technique (e.g. structure interview) is going to help you gather the correct data and How are you going to analyze the data for addressing your problem. You can also mention the expected outcome of your study and how will it be relevant to the stakeholders. Good Luck