Answer:
The correct answer is: "Sí, ellos son nuestros abuelos" ("Yes, they are our grandparents").
Explanation:
The correct word to use when referring to the parents of someone's mother or father is grandparents, which in Spanish is "abuelos". So, when Marta asks wether they are their mother's parents, we should use this word instead.
Now, Marta is asking wether they are Marco and Juan relatives, then the answer has to include a possessive pronoun, such as mine, our, its, her, etc. But as Marta is asking if they are related to two people (Marco and Juan) then it has to be plural possessive pronoun, in this case our ("nuestro"), for the question is directly to the ones involved.
But, even if in English we use our for plural or singular nouns, in Spanish changes according to number of the noun. In this case as the noun is "abuelos", that is plural, then possessive pronoun has to be plural too, and for that reason is "nuestros".
This leave us with the correct answer that would be: Yes, they are our grandparents, or in Spanish: "Sí, ellos son nuestros abuelos".
