4. In your career, you will have to deal with many ethical issues. What are likely to be the most challenging and what is your plan for developing the competencies you will need to handle these issues effectively? (400-word limit)
They are asking for "what are likely to be the most...", you don't have to draw from prior work experience if you have none. If you have student government/leadership experience, you can use those as basis as well. Just think about what would be the most difficult water to tread and how can you prepare for it. Use your imagination!
When you write this essay, you must consider what is your career goal? What kind of ethic issues you will encounter in your career path? Then do some research of HBS how MBA program can help you ace the ethic issues.