The quiz asks you to use the gerund to say while (doing something). To get rewarded full points for this mini quiz however, you also have to select the option that uses mientras. If the question had been something like, "select the correct ways to say while (doing something)" it would have been clearer to me.
I checked the questions related to this lesson and can't really see that it's asking for a specific form/structure. The translations in English are using the most common/natural way to translate the gerund that we use in Spanish for this purpose, which is "while doing something". The lesson is explicitly saying that you can say this either using the gerund or also using mientras with a conjugated verb. This is why we include the two possible answers.
I hope it clarified it.
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