Final answer:
The most appropriate form of the verb to fill in the blank is 'llegó.'
Step-by-step explanation:
In the sentence, 'al teatro a las siete y media anoche,' the verb 'llegó' is the most appropriate form to fill in the blank. This is because it agrees with the third person singular subject, which is not specified in the question. 'Llegó' is the preterite tense form of the verb 'llegar,' meaning 'to arrive.'
Learn more about Verb conjugation in Spanish