Final answer:
The appropriate preterite verb to fill in the blank is 'se cayó' which means 'fell'
Step-by-step explanation:
The appropriate preterite verb to fill in the blank in the sentence is A. se cayó. The sentence can be translated to English as 'Mariana fell because she didn't see the stairs.' In this sentence, the verb 'caer' (to fall) is conjugated in the preterite tense to match the subject 'Mariana' and the past action of falling.
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