Final answer:
The correct form of the verb tener to complete the sentence is 'tienen'.
Step-by-step explanation:
The correct form of the verb tener to complete the sentence 'Ellos un primo en Cuzco' is 'tienen'.
In this case, 'Ellos' is the subject of the sentence, which is the third person plural pronoun meaning 'they'.
Therefore, the correct answer is 'tienen' which is the third person plural form of the verb 'tener'.
Learn more about Verb conjugation in Spanish