Right answer: nos
This is a good example of the usage of the Direct Object, which is the part of the sentence that directly receives the action of the verb. It is used to replace a person, animal or thing in the conversation or in the writing in order to avoid repetition; when they have already been seen in a sentence or external context where the statement occurs.
In this case who directly recieves the action of the verb pregunta (to ask) is nosotros (us), hence nosotros is the direct object.
Now, in order to substitute this direct object we can use the Direct Object Pronouns, which are written before the verb, when it is conjugated.
Here the pronoun nosotros is the 1st person plural and its direct object pronoun is nos.
Therefore, the complete sentence is:
Sofía nos pregunta (a nosotros)
Sofia asks (us)
