Final answer:
The verb 'hacer' is in the imperfect tense because it expresses a repetitive action in the past.
Step-by-step explanation:
The verb hacer is in the imperfect tense in the sentence 'Cuando era pequeña, Marta hacía travesuras' because it expresses a repetitive action in the past. The imperfect tense is used to describe ongoing or habitual actions in the past, while the preterite tense is used for completed actions. In this case, Marta used to make mischief repeatedly when she was young.
Learn more about imperfect tense in Spanish