Final answer:
The correct verb form that completes the sentence is 'creyeron.'
Step-by-step explanation:
The correct verb form that completes the sentence is creyeron. In the sentence, 'Ellos siempre...que América debía ser liberada,' the subject 'Ellos' is plural, so we need to use the plural form of the verb 'creer,' which is 'creyeron.' This means the correct answer is option C.
Learn more about verb forms in Spanish