I think the answer is easy....

one part of it it, that is. The prompt says "Describe a situation"... that means 1. The article is important and I think the AdCom are looking for just one situation but they want you to go deep and provide details. You can tell a lot with one story and show yourself from many angles.
As to which story and which main angle to pick, it is a personal choice that depends on your application and your background. If you want to change into a new field, e.g. you want to switch into Marketing for example, and your resume is all about Finance, you don't want to talk about Finance in your story but marketing or trying a new thing and that it really made you feel confident about the transition, etc, etc.
Another way to choose the topic/story is based on your weaknesses, concerns that the adcom may have about your profile. You may either want to reinforce something (e.g. you had a very low undergrad GPA or low EA scores) or perhaps you want to show that you are better than what you resume says (e.g. you lack leadership words/experience on the resume). You can also think about which of the stories represents your essence. Is that you as a problem-solver, you as a peace-maker, or you as a go-getter. There are many ways to think about it.
-b
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