I think the best approach is to reach your own conclusion about the meaning of part a).
I would take part a) at face value. Part a) is part of Question 1 (Professional Aspirations). Question 1 includes the general MBA questions (Why now? What are you long term and short term goals?). Thus, part a) appears to be the basic autobiography prompt i.e. pick some important life event(s) that you want to share with the admissions staff. The idea is to provide a strong link from your previous choices to how an MBA will help you now, to how a Stern MBA will continue to serve you in the future. The path of least resistance is to use professional experiences in a) as a springboard to parts b) and c) but one could use personal experiences (especially if the applicant has low work experience).