Final answer:
The correct verb form that completes the sentence is 'han leído'.
Step-by-step explanation:
The correct verb form that completes the sentence is han leído.
This is because the subject of the sentence is 'Ellos', which is the third person of the plural form. To match this subject, we need to use the third person plural form of the verb. The verb form 'han leído' is the correct conjugation for the verb 'leer' in this case.
For example, if the subject of the sentence were 'Tú' (you), then the correct verb form would be 'has leído'.
Learn more about conjugation of verbs in Spanish