Answer: The correct form of the verb to complete the sentence "Eric et Marc ________ leurs livres" is "vendent".
Step-by-step explanation:
In French, verb conjugations change depending on the subject of the sentence. The subject "Eric et Marc" is a plural third-person subject, so we use the plural form of the verb "vendre", which is "vendent". This form matches the subject and completes the sentence correctly.
Therefore, the correct sentence is: "Eric et Marc vendent leurs livres."