Answer:
Mariana se cayó porque no vio la escalera.
The answer would be "se cayó" because that means "fell down"
For, B te caiste if someone was saying it to another person. For A se caía it doesn't make sense to put it in the sentence because it wouldn't sound good.
For C se cea would make the sentence sound and you use it for a different sentences.
So "se cayó" is the best answer
Hope this helps
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