Final answer:
The appropriate verb to fill in the blank is 'se cayó' in the preterite tense, indicating a completed past action.
Step-by-step explanation:
The appropriate verb to fill in the blank in the given sentence is se cayó. The sentence translates to 'Mariana fell because she did not see the stairs.' In this case, the verb 'se cayó' is in the preterite tense, indicating a completed action in the past. Other options like 'se cae' (present tense), 'te caiste' (informal second person), and 'se caía' (imperfect tense) are not suitable in this context.
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