Answer:
Fuiste
Step-by-step explanation:
Ir is the verb meaning "to go".
It is conjugated below for the preterite, or past tense.
Yo fui
Tú fuiste
Él / ella/ usted fue
Nosotros fuimos
Vosotros fuisteis
Ellos/ ellas /ustedes fueron
We are given the sentence:
Tú _____________ (ir) a la fiesta.
We need to conjugate ir for tú in the past tense. If we look at the conjugated verbs above, we see that fuiste is the correct choice.
Tú _____________ (ir) a la fiesta. --> Tú fuiste a la fiesta (You went to the party).