Schools that ask this kind of question are not trying to trick you or judge you by your choice, rather they are simply giving you the freedom to present the best case you can for your candidacy. Therefore, you should do any or all of the things you describe under the framework of:
how can I best present evidence for admission? I would go back over my application and ask myself if I adequately demonstrated the "four pillars" of a good applicant: leadership, maturity, innovation and teamwork. Any area that you feel could use some "shoring up," you could detail that area here. Certainly if you have a particular strength or an overarching reason that is compelling and could benefit from another example, you could also go that route. They are trying to get to know you, so open up and let it fly!