Final answer:
The correct indirect object pronoun in this sentence is 'nos'
Step-by-step explanation:
The correct answer is b. nos.
In this sentence, Alberto's younger twin sisters are the indirect object recipients of the action of the verb 'enseña' (teach). The pronoun 'nosotros' represents 'Alberto's younger twin sisters' and serves as the indirect object of the verb.
For example, if we rephrase the sentence using the indirect object pronoun 'nos,' it would be: 'A veces, la profesora de matemáticas nos enseña a bailar'. This translates to 'Sometimes, the math teacher teaches us to dance.'
Learn more about Indirect object pronouns in Spanish