Correct answer:
b. si es el perro nuestro
Both nuestro and vuestro are possessive adjectives. Possessive adjectives are used to indicate ownership or possession. They agree in both gender and number with the thing possessed, not the possessor. So vuestro is the possessive adjective for the second person plural, so someone asks this question to us. Therefore, we must answer using the possessive adjective for the first person plural, which is nuestro. Finally:
¿Es vuestro perro?
Sí es el perro nuestro