Final answer:
The correct form of the verb ser to use in the sentence is 'son' as the subject is plural.
Step-by-step explanation:
The correct form of the verb ser to use in the sentence 'Mis mascotas ___ un perro y un gato' is son.
The subject of the sentence is 'mis mascotas,' which means 'my pets' in English. Since the subject is plural, the verb ser should also be in the plural form, which is 'son.'
Example: Mis mascotas son un perro y un gato. (My pets are a dog and a cat.)
Learn more about Verb ser in Spanish