Final answer:
The correct verb form in this sentence is 'vio' in the preterite tense.
Step-by-step explanation:
In this question, we are asked to complete the sentence with the correct verb form in the preterite or imperfect tense. The correct answer is a. vio.
The preterite tense is used for completed actions in the past, while the imperfect tense is used for ongoing or habitual actions in the past. In this case, the verb 'ver' (to see) is a completed action, so we use the preterite tense 'vio'.
Therefore, the correct sentence is 'De repente el oso vio al niño'.
Learn more about Spanish verb conjugation