Final answer:
The correct conjugation is 'creíamos' which means 'we used to believe'
Step-by-step explanation:
The correct conjugation of the verb in parentheses to complete the sentence is creíamos.
When we were children, we used to believe in Santa Claus.
Using the imperfect tense 'creíamos' indicates an ongoing action, suggesting a regular belief in Santa Claus during their childhood.
Learn more about Conjugation of Spanish verbs