This sentence uses 女の子, but couldn’t 女子 or 少女 work all the same?
Not a native speaker, but according to my intermediate (~N2) Japanese knowledge, yes - those would both work as well!
Is she ‘practicing the action’ of be a girl? (The never-ending は vs. が battle)
女の子、女子 and 少女 are not always interchangeable.
あなたが女の子だからよ。= That’s because you are a girl. (i.e. because of her gender, not her age)
あなたが女子だからよ。= That’s because you are female. (i.e. “you” might be an adult female)
あなたが少女だからよ。= That’s because you are a naive little girl. (i.e. emphasizing her immaturity)
A ladies’ room is 女子トイレ (じょしトイレ): all females at any age are allowed to use it. We don’t call it 女の子トイレ nor 少女トイレ.
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