<h2>Answer: escribirán</h2>
This sentence written in future is related to a book <u>Carlos and Sofía </u><u>(they)</u> <u>will write</u> before leaving Puerto Rico.
In this context, the correct conjugation of the verb escribir (to write) in future (<u>indicative mood</u>) with the <u>3rd person in plural</u> ellos (they), is escribirán (they will write).
Therefore, the complete sentence is:
<h2>Antes de salir de Puerto Rico Carlos y Sofía <u>escribirán</u> un libro sobre la historia de Puerto Rico.</h2>
Carlos and Sofía <u>will write</u> a book about the history of Puerto Rico before leaving Puerto Rico.
