Final answer:
The correct conjugation of the verb 'ser' in the imperfect tense to complete the sentence is 'eran'.
Step-by-step explanation:
The correct conjugation of the verb 'ser' in the imperfect tense to complete the sentence is eran. This form of the verb is used to describe actions that were ongoing or habitual in the past. In this case, it is used to describe the dinosaurs as 'being' or existing as very large animals.
Learn more about Conjugation of the verb 'ser' in the imperfect tense