Final answer:
The appropriate verb to fill in the blank is 'fueron' as 'Ellas' is plural and the action happened in the past.
Step-by-step explanation:
The appropriate verb to fill in the blank in the sentence is fueron.
This is because the subject Ellas is plural and the action of going to the supermarket happened in the past, indicated by the word anoche.
Therefore, the correct conjugation of the verb ir in the past tense for the subject Ellas is fueron.
Learn more about Spanish verb conjugation