Answer:
La semana que viene Carlos y Sofía escribirán una carta a su abuelo para contar todo lo que están aprendiendo en Puerto Rico.
Explanation:
To be fair, we could use a lot of verbs here, but "escribir" (to write) is probably the most appropriate. Since it's a future tense and the noun is third person plural, the correct form of the verb is "escribirán".
