Final answer:
The direct object pronoun that substitutes the underlined word 'us' is 'nos'.
Step-by-step explanation:
The direct object pronoun that substitutes the underlined word 'us' in the sentence 'They are waiting for us' is nos. In Spanish, the pronoun 'nos' is used to replace 'us' when it functions as the direct object of the sentence. For example, instead of saying 'Ellos están esperando a nosotros', we can say 'Ellos están esperando a nos'. This pronoun shows that the action of waiting is directed towards 'us'.
Learn more about Direct object pronouns in Spanish