Final answer:
The correct word to complete the sentence is 'existieron'. 'Existieron' is the past tense form of the verb 'existir' in Spanish.
Step-by-step explanation:
The correct word to complete the sentence is existieron.
'Existieron' is the past tense form of the verb 'existir' in Spanish, which means 'to exist'.
In this sentence, 'Los dinosaurios' is the subject and it agrees with the verb 'existieron' in number and gender.
Learn more about Completing a sentence in Spanish