Final answer:
The verb that should be filled in the blank is 'hablan'
Step-by-step explanation:
The correct verb to fill in the blank is D. hablan.
This is because 'el plano' is the subject of the sentence, which is a third person plural pronoun, therefore requiring the verb 'hablan' to agree with it.
For example, you could say 'Ellos hablan el plano todos los días' which means 'They talk about the plan every day'.
Learn more about Verb conjugation in Spanish