Final answer:
The correct verb form to fill in the blank is 'hablan'.
Step-by-step explanation:
The correct verb form to fill in the blank in the sentence is hablan. The subject 'Ana y Mariana' is a plural third person, so we use the verb form 'hablan' to match. In this sentence, Ana and Mariana are speaking on the phone, so we can complete the sentence with 'por teléfono'. Therefore, the correct answer is hablan por teléfono.
Learn more about Verb Conjugation