Final answer:
The verb form 'hablan' should be used to fill in the blank in the sentence.
Step-by-step explanation:
The correct verb form to fill in the blank in the sentence is hablan. It is the appropriate form for the subject 'Ana y Mariana' which is third person plural. The verb 'hablar' means 'to speak', and the correct conjugation for third person plural is 'hablan'.
Learn more about Verb conjugation in Spanish