Final answer:
The correct form of the verb 'bailar' to fill in the blank is 'bailaba'. It represents an ongoing or habitual action in the past.
Step-by-step explanation:
The correct form of the verb 'bailar' to fill in the blank in the sentence 'Cuando yo llegué a la fiesta, Ana ... con Juan' is bailaba (option D).
The verb 'bailaba' is the imperfect past tense form of 'bailar' (to dance) in Spanish. It is used to describe an ongoing or habitual action in the past. In this sentence, it suggests that Ana was already dancing with Juan when the speaker arrived at the party.
Here is an example sentence using the same verb form: 'Antes de la pandemia, yo bailaba salsa todas las semanas.' (Before the pandemic, I used to dance salsa every week.)
Learn more about Spanish verb conjugation