Question

Les filles ______ ___________ en retar(d) au théâtre. (arriver)

Answer 1

Sont arrivées 


This is because "arriver" is a MRS. VANDERTRAMP verb. meaning that it conjugates with être in the perfect tense.

Here is a list of the MRS. VANDERTRAMP  verbs:

Montre
Retourner
Sortir

Venir
Arriver--- arrivé + es(because you are conjugating for "elles")
Naître
Desendre
Entrer
Rester
Tomber
Rentrer
Aller
Mourir
Partir

Bonne Chance!

Answer 2

I'm not sure what verb tense you wanted, but since there's two blanks to fill, i thought you would want it to be in passé composé.

The answer is "sont arrivées."