Final answer:
The correct imperfect conjugation of the verb 'ir' is 'iba' to fill in the blank in the sentence.
Step-by-step explanation:
The correct imperfect conjugation of the verb 'ir' to fill in the blank would be 'iba'.
So the correct sentence would be: Ella asistía a la clase pero no fue.
'Iba' is the imperfect tense form of 'ir' and is used to describe ongoing or repeated actions in the past. In this context, it indicates that she used to attend the class but didn't go on that particular occasion.
Learn more about Spanish verb conjugation