Final answer:
The correct answer is 'bailaban'.
Step-by-step explanation:
The correct answer is A. bailaban.
In this sentence, the verb bailar refers to Beatriz and Ignacio, which are the third person plural subject pronouns. Therefore, we use the third person plural form of the preterite or imperfect tense, which is bailaban.
For example, the sentence could be translated as 'Beatriz and Ignacio were dancing once during the party.'
Learn more about Spanish verb conjugation