5. A María y a Néstor les gustan los primos. (Su/Sus) prima mayor se llama Ariana.

Answer

The correct choice is 'Su'. In Spanish, possessive adjectives must agree in number (singular or plural) with the noun they modify, not with the owner. Even though there are two owners (María and Néstor), the noun being possessed is 'prima mayor' (oldest cousin), which is singular. Therefore, the singular possessive adjective 'Su' is required. Using 'Sus' would be incorrect because it would imply that there is more than one oldest cousin being referred to.