Sourabh Goyal , you should first see if you are going to study NPAs in Retail Loans as HDFC Bank is mainly in retail banking. You should examine what you want to study: The reasons for NPAs or the mechanism to control NPAs. So your questionnaire should be based on objectives of your research.

Basically, on Non-Peforming Assets, we do research on Secondary Data by measuring the trend of Gross NPA, Net NPA etc but since you stated what types of questions you should ask.. so your study must be on the basis of opinion. but still, it confuses me what will be your respondents? Mangers or Customers. All your questions will be based on the purpose and to whom you are asking. Please clarify so I can respond Properly.

You need to check the literature review & theoritical framework

They are the best places to find an answer for your question

All the Best

Your choice of variables and analytical technique form the bases of the choice of the questionnaire. No researcher can from space inform about the nature of questions except with a clear view of your conceptual framework or analytical method.