Final answer:
The most appropriate form of the verb to fill in the blank in the given sentence is 'llegué.' It is the first person singular preterite form of the verb 'llegar' (to arrive) in Spanish.
Step-by-step explanation:
The most appropriate form of the verb to fill in the blank in the given sentence is llegué.
The correct sentence is 'Yo llegué al teatro a las siete y media anoche.'
'Llegué' is the first person singular preterite form of the verb 'llegar' (to arrive) in Spanish, which matches with the subject 'yo' (I) in the sentence.
Learn more about Spanish verb conjugation